# Apogalacticon and Perigalacticon

What is the length of the apogalacticon and perigalacticon of the Sun and Milky Way? The general terms seem to be apoapsis and periapsis.

My greatest efforts at Googling have failed miserably. If you can provide references as well, please do!

According to this website we need another 15 million years to the perigalacticon.

Recall the Sun's motion… 30 degrees toward the galactic centre from circular motion, and 23 degrees upward out of the galactic plane. We will be at perigalacticon (closest to the nucleus) in 15 million years. At apogalacticon the Sun is seven percent farther out.

During the galactic year of about 250 million years our solar system isn't orbiting on a Kepler ellipse around the galactic center, but instead oscillating up and down the galactic plane.

This vertical oscillation cycles at 3.5 times per galactic year.

We are now about 27,000 lightyears away from the galactic center, close to the perigalacticon. The difference between apogalacticon and perigalacticon is a little more than 4,000 lightyears (15% of 27,000 lightyears). The 15% number is calculated from the 7% eccentricity of the orbit by $1.07/0.93=1.15~~$ according to the definition of eccentricity, and Kepler's 1st law.

user68441 is in error. We are not part of the Sagittarius Dwarf Elliptical Galaxy. Our velocity perpendicular to the galactic plane is only 4.95±0.09 km/s with respect to the Local Standard of rest.

Each cycle takes about 70 million years with an amplitude of 100pc (Matese et al. 1995), it will be roughly 30 million years before we cross the plane again.

Given the thickness of the disk is roughly 1000LY, the solar system will stay within the disk, unless perturbed by an interaction with another star.

## Apogalacticon and Perigalacticon - Astronomy

This article presents and discusses a measurement of the proper motion for the Carina dwarf spheroidal galaxy (dSph) from images in two distinct fields in the direction of Carina taken with the Hubble Space Telescope, at three epochs. Each field contains a confirmed quasi-stellar object that is the reference point for measuring the proper motion of the dSph. The consecutive epochs are 1-2 yr apart. The components of the measured proper motion for Carina, expressed in the equatorial coordinate system, are μ α =22+/-9 mas century -1 and μ δ =15+/-9 mas century -1 . The quoted proper motion is a weighted mean of two independent measurements and has not been corrected for the motions of the Sun and of the local standard of rest. Given the proper motion and its uncertainty, integrating the family of possible orbits of Carina in a realistic gravitational potential for the Milky Way indicates that Carina is bound gravitationally to the Milky Way and is close to apogalacticon. The best estimate of, and the 95% confidence interval for, the apogalacticon of the orbit is 102 kpc and (102,113) kpc, for the perigalacticon is 20 kpc and (3.0,63) kpc, and for the orbital period is 1.4 Gyr and (1.3,2.0) Gyr. Carina does not seem to be on a polar orbit. The best estimate of the inclination of the orbit with respect to the Galactic plane is 39°, but the 95% confidence interval is so wide, (23°,102°), that it includes a polar orbit. We are unable to confirm or to rule out the membership of Carina in a stream'' of galaxies in the Galactic halo because the difference between the observed and predicted directions of the proper motion is 1.6 times the uncertainty of the difference. Carina must contain dark matter to have survived the tidal interaction with the Milky Way until the present. The triggering of star formation by perigalacticon passages and crossings of the Galactic disk do not explain the history of star formation in Carina.

Based on observations with NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.

## Galactic Evolution: Cycling From Bar>Normal Spiral

In the article, she very succinctly explains how a galaxy can cycle from being a normal spiral to a barred spiral and back to a normal spiral during its life.

It explains why a spiral doesn't have its arms "wrap up", and explains how images of barred and normal spirals are merely "snapshots" in the life of a galaxy.

What I'd like to know is why this has been ignored by other authors. Though published in Scientific American (where the readership is huge), most books and articles written on galactic evolution since this article was published blithely ignore the explanation she elucidates. Even recent articles in Astronomy and Sky & Tel explain galactic form and evolution as if her ideas were never stated or explained.

Yet I find her explanations most compelling and convincing, even though I have read hundreds of articles and books on galactic evolution and star formation.

I have to believe that most people must have missed the article or that she is somehow the Halton Arp of our generation, and shunned by other members of the astronomical community.

Whatever the truth about why this article hasn't created more waves than it has, I wanted to bring it to the attention of CN readers.

### #5 deSitter

Whence the bar? That's the key question. And where do ellipticals fit in? This is a good thing, to see morphology take a front seat again, but there is a long way to go to a real theory of galactic evolution.

### #6 Starman1

The article explains how bars result.
Ellipticals, I guess, are still unknown.
Large ones could result from mergers, which would also explain their lack of gas and the age of the stars. I've seen computer simulations which show how an elliptical grows from mergers. It would explain why so many of them are large, near the center of galaxy clusters, and, possibly, older.
Small ellipticals, though, I haven't seen a good explanation for.
One morphological study showed that the large spirals often had their north-south poles perpendicular to the large "bubbles" in the galactic distribution studies. That, too, was a revelatory lucubration (a great word for astronomy, since the root is "bringing light to").

You're right, though, I get the impression we haven't come a long way from Hubble's "Tuning Fork".

### #7 Jay_Bird

This reminds me of one of the most memorable articles I read as an earth science undergrad, maybe 'IlaniteDave' will recall it, by Robert Folk (an icon of sedimentary geology): Rollers and Ripples in Sand, Streams and Sky, from 1976. He talked about transitions from longitudinal to transverse vortices at many orders of scale from sand ripples to galaxies (he saw the same type of "tuning fork" [not Hubble's tuning fork] shaped junctions in ripples at all those scales), and based on keen observation and a little 'arm-waving' over many illustrations it held together very well. His reputation led to publishing in J. Sedimentology or J. Sedimentary Petrology, with a vigorous comment and response. For months I was looking for longitunal / transverse changes and saw them as thickness changed in films of paint being rolled on walls, and in frying pan grease, as well as in gutter, river, and lake sediments.

Not trying to hijack the thread, just recalling the excitement of a similar 'big thinking' article as a quickening breath of academic fresh air.

### #9 deSitter

The article explains how bars result.
Ellipticals, I guess, are still unknown.
Large ones could result from mergers, which would also explain their lack of gas and the age of the stars. I've seen computer simulations which show how an elliptical grows from mergers. It would explain why so many of them are large, near the center of galaxy clusters, and, possibly, older.
Small ellipticals, though, I haven't seen a good explanation for.
One morphological study showed that the large spirals often had their north-south poles perpendicular to the large "bubbles" in the galactic distribution studies. That, too, was a revelatory lucubration (a great word for astronomy, since the root is "bringing light to").

You're right, though, I get the impression we haven't come a long way from Hubble's "Tuning Fork".

There has to be something first to spiral up! Meaning the bar comes first. One sees in the deep fields many spindle-shaped galaxies. Some of these are lensed, but there are far too many of them to attribute just to lensing or foreshortening. Then there are simple bars with trailing wings, no spiraling. One sees bars and rings together. The galaxies are too slow moving and too distant to much interact with each other on the scale of the winding.

I'd like to see the models. I would like to be sure relativity, special and general, is being used correctly.

### #11 deSitter

The black holes came first. OK.

### #12 llanitedave

This reminds me of one of the most memorable articles I read as an earth science undergrad, maybe 'IlaniteDave' will recall it, by Robert Folk (an icon of sedimentary geology): Rollers and Ripples in Sand, Streams and Sky, from 1976. He talked about transitions from longitudinal to transverse vortices at many orders of scale from sand ripples to galaxies (he saw the same type of "tuning fork" [not Hubble's tuning fork] shaped junctions in ripples at all those scales), and based on keen observation and a little 'arm-waving' over many illustrations it held together very well. His reputation led to publishing in J. Sedimentology or J. Sedimentary Petrology, with a vigorous comment and response. For months I was looking for longitunal / transverse changes and saw them as thickness changed in films of paint being rolled on walls, and in frying pan grease, as well as in gutter, river, and lake sediments.

Not trying to hijack the thread, just recalling the excitement of a similar 'big thinking' article as a quickening breath of academic fresh air.

## The Fornax Dwarf and its Globulars, Part II

Above: from An Atlas of the Universe, The Local Group , © Richard Powell . Current evidence shows that every dwarf galaxy in the Local Group has had a different star formation history from every other. Most of the dwarfs shown above are in orbits that take them over the polar halo regions. Note how distant many are from the galactic disk. This view includes the eleven new SDSS-discovered dwarfs since 2000, e.g. Pegasus , Phoenix , Pisces , and Antlia ( 1 , 2 ).

Dwarf galaxies are the building blocks of larger galaxies like our Milky Way. But how did dwarf galaxies get built? The Fornax Dwarf may be the brightest dwarf galaxy we can see with our telescopes, but it belongs to a class of galaxies, the dwarf spheroidals (dSph for short), that are the least luminous of the galaxy types. “Least luminous,” though, does not mean “least massive.” Inferring from the motions of their stars , astronomers have found that dSphs typically exhibit a total mass many times that which can be accounted for by the mass of the stars themselves. Put another way, dSphs have very high mass-to-light ratios —100 to 1000 times as much mass as luminosity is not unusual. (See 4.1.2 “Does Mass Follow Light” starting on p.27 here .)

Dwarf galaxies of all kinds, but especially dwarf spheroidals like Fornax, have thrown open an imposing number of new doors into the galactic study hall—hence the need for this provocatively titled article submitted to arXiv.org only four months ago: ”’ Galaxy,’ Defined ” by Beth Willman and Jay Strader . (Willman has the distinction of a new galaxy being named after her ), The Fornax Dwarf of interest here is an inconspicuous galaxy which is fun to track down in our telescopes. It looms much more conspicuously in the eyes of observatory astronomers. Until 2000, life was a tame affair among astronomers specializing in dSph studies. Nine small galaxies in the Milky Way Group comprised the lot. They all contained little or no gas and dust, but most exhibited evidence of long star formation histories e.g., the presence of carbon stars and sizable red giant branch populations in their makeup. The remote Fornax and Carina dSphs had experienced several starburst episodes whose gas was hard to account for, while the Ursa Minor Dwarf formed early and stayed quiet ever after. (Nowadays the UMin Dwarf is in the spotlight again precisely because it did not have starburst episodes its color-magnitude diagram resembles the globular M92 and NGC 2419 the Intergalactic Tramp more than its other dSph cousins.) This is a feature with several dSphs that tends to blur the morphological distinction between the two. The big difference is dark matter, abundant in dSphs but absent in globulars. Three of the new discoveries— Ursa Major II , Willman 1, and the Coma Berenices Dwarf— had measurable masses of over a million suns, but shone only as brightly as a thousand suns . The Draco Dwarf, 70 times fainter than Fornax, has a dark matter halo five times as massive as Fornax’s.

In 2000 came the first Sloan Digital Sky Survey data and everything changed. Eleven new dSphs sprang into scrutiny, along with tantalizing very low surface brightness “orphan” streams in the MW’s outer halo that hint our galaxy conceals many shadows of forgotten ancestors. The new SDSS brood exhibited features common to known dSphs : little or no residual gas very low present star formation rates highly inclined orbits that took them high above and below the galactic plane high mass-to-light ratios metallicities midway between globulars and dwarf ellipticals (and about a third of the Sun’s*) and a red giant branch with few RR Lyrae stars. Astronomers dating their earlier starburst episodes found that starbursts tended to coincide with their closest passages through a parent galaxy disk. “Through” in the case of Fornax meant the galaxy is still 180,000 light years out from the visible disk’s outlier stars. Star counts and spectroscopy revealed that many dSphs had bar-like features across the axis of their own rotations. Bars in galaxies funnel—or rather, mass-transfer—gas in towards the core. This turned out to be significant when astronomers computer-modeled the Local Group’s early development .

*Most core-collapse SN ejecta are roughly solar metallicity.

Thus, astronomers could clearly demark dSphs as a different breed of galaxy than dwarf ellipticals (dEs) or massive globulars like Omega Centauri or Mayall II in M31. Like globulars and dEs, dSphs are small, mostly ancient agglomerations of stars. “Small” is a relative term: it can mean 50 million or more stars in a dwarf elliptical , or as few of 10,000 in a case like the globular Pal 12 , which is gradually being stripped of its stars by the Milky Way. “Ancient” likewise is a relative term. Most globulars formed very early in the universe out of primordial gas, without the influence of dark matter. By the time large galaxies began to form out of the same lumps of primordial gas, globulars were cohesive enough to resist the gravitational siren song of galactic disks. Meanwhile, dark matter was itself busily clumping along the same cusps that spawned so many galaxies. Globulars thereby missed the dark-matter boat. In contrast, most dEs early dwarfs were gas-poor, metals-poor agglomerations of smaller galaxies—usually gas-rich, metals-poor dwarf irregulars (dIrr). NGC 4449 in Canes Venatici is currently nibbling away at one of its dIrr companions (so small it falls under the recently coined term “ Hobbit Galaxy ”). IC 2574 “Coddington’s Nebula” has a tiny galaxy infalling toward it. NGC 147 and 185 in Cassiopeia are dEs that can be spotted in 100mm scopes in good skies whose globular metallicities resemble four of the Fornax dSph globulars. The Small Magellanic Cloud is generally considered to be a dwarf irregular, as is NGC 6822 Barnard’s Galaxy. DIrr’s have quite a brood of pals: 22 of the 41 galaxies in the Local Group are classified as dIrrs.

Quantifying the distinction between the various dwarf types is an involved process. Fifteen years ago a study by Jorgensen and Jimenez (A&A, 1997, vol. 317, p. 54-64) performed photometric observations of the giant and horizontal branches of the globular Fornax 1, and of the giant branch of Fornax 3 (NGC 1049). The team used two narrow-band filters especially designed to distinguish between carbon and M type stars. They did this to weed out “carbon chatter” from the intervening Milky Way carbon and M stars, as well as enriched MW gas floating between large clusters and clouds. The red-giant branches of the Fornax clusters are much broader than the same branches in Milky Way globulars. Jorgensen and Jimenez concluded, “We suggest that the morphology and stellar population of the giant branches indicate that the dwarf galaxies are =

3Gyr younger than the Galactic halo, which in turn seems to be =

3Gyr younger than the Galactic globular clusters.” Check these figures against other papers in the literature, notably p.6 in this paper , and you can see how quickly new instruments and data processing can revise estimates only a decade old. (The preposition “about” is a handy hedge in the astrophysical glossary.)

There is an additional wrinkle in the chemistry of dwarf galaxies like Fornax and Draco which are remote from their host galaxies: iron chatter. The mean iron abundance observed in the intergalactic medium (IGM) of the Local Volume (defined as the surface at which there is no net motion into or out of the Local Group, usually taken to be 48.75 Mly—interesting graphic of 248 components here .) contains nearly the same amount of iron per unit volume as the intergalactic medium of the Universe as a whole. The iron was produced by supernovae exploding at a rate proportional to the cosmic star-formation rate. If most SNe occur inside galaxies, then iron ratios in the IGM would have been altered by galactic winds and jets sweeping galactic gas containing SN 2b metals. Later-forming dwarf galaxies like Fornax are known to have been “pre-enriched” with iron at levels consistent with the density structure of the interstellar medium . A somewhat overlooked 2007 paper in Astrophysics ( 662:322-334,2007 ) points out that the Milky Way is rather a “quiet” galaxy compared with M31 and other members of the Local Group. (See this paper for Local Volume dwarf galaxy starburst activity.) A fraction of the early SN iron found in Fornax and other remote dwarfs cannot be attributed to tidal interactions, mass loss through starburst winds, SN ejecta, and jets hence the term “iron chatter”.

Of the four types, only dSphs have significant amounts of dark matter. This has a decisive impact on their star formation histories. Most papers in the professional literature agree that each of the dSphs we see occupies only a small stellar core in a giant dark-matter halo these range from 100 million to one billion solar masses. Dark matter and metallicity are the chief points of distinction between dSphs and dEs. In contrast, dEs have much more visible matter in proportion to dark—they are basically youthful versions of M87 in Virgo. But unlike giant ellipticals, dwarf ellipticals and sphericals rotate as unitary bodies in addition to their seething internal star motions.

Dsphs, in short, are unique unto themselves, and the Fornax Dwarf is unique among them. The basic scenario of dSph star formation is a punctuated equilibrium or starburst model. The Fornax Dwarf’s evolution certainly has been bumpy. Its primordial gas condensed into stars around 10 billion years ago in a burst that dwindled slowly into a genteel pace of three new stars every 10,000 years. (By compare, 10,000 years is roughly the entire span of humankind’s agriculture-based history.) Star formation has occurred in several sharp bursts. In massive spiral galaxies, the short intervals between star formation peaks mimics a continuous star formation rate. Less massive systems like Fornax exhibit chronologically distinct starbursts. The delay between the star-forming bursts is controlled by the gas cooling time, which depends on the galaxy’s mass and distance from the hot halo gas of the Milky Way, which depends on, of all things, Kepler’s equations. More about this a little later on.

What’s in all this for us, the astrophiles without million-dollar telescopes?

Challenging sightseeing, for one. Both dSphs and dwarf ellipticals can have globular clusters. The Fornax Dwarf has five the first report in this series dealt with them. Alvin Huey’s The Local Group (p.32-33) identifies four globulars in NGC 185 for us to chase after, and another four in NGC 147. These globulars placidly orbit their parent galaxies gigayear after gigayear, dark matter or no dark matter.

But why do dSphs like Fornax have so much dark matter? Where did it come from? How did they acquire or evolve in association with it? Why doesn’t the presence of, say, a billion solar masses of dark matter surrounding them not especially perturb the orbital efforts of a family of globular clusters with a piffling mass (by dark matter standards, anyway) of 25,000 to 150,000 stars? Why don’t they sink into the middle of the galaxy? Over time, the astronomers’ bepuzzled, “Why haven’t they fallen into the middle?” became “Why are they so stable?”

Star-Forming Episodes: The Coddington Analogue

It may look tame now, but what did the Fornax Dwarf look like during its several star forming periods? Since the dSphs are characterized by low average star formation activity, to picture what they were like in the past we have to use analogues. Luckily, a conveniently analogous starburst dwarf exists for Northern Hemisphere observers. Recently the Rome-based astrophotographer Leonardo Orazi posted an image of the dwarf irregular galaxy IC 2574 Coddington’s Nebula in the M81 Group as it undergoes a tempestuous enrichment cycle amid a burst of star formation. (The word “Nebula” is a misnomer it was unresolvable when discovered in 1898.) The Hubble website also has featured it recently.

IC 2754’s 48 supernova bubbles illustrate how supernova explosions ignite cycles of star formation along the rims of the holes the supernova created. Coddington’s star-forming shells range from under 300 ly to over 3,000 ly in diameter.

Star-forming bubbles in IC 2574. [Source: Weisz et al. ] The smallest (#44) is 600 ly in dia. The largest (#13) is 6240 ly in dia. It has enough mass to make 14.5 million suns. “Swiss cheese” is an inapt analogy for this structure cheese bubbles are rigidly trapped in an inflexible medium. Rather, it’s more like looking down into an effervescing column of sparkling water. Now imagine it is 6 billion years ago and you are looking at the Fornax Dwarf. The center is ablaze with young star-forming regions erupting huge amount of O and B stellar wind and, soon enough, supernova shock fronts. (Proof, if we needed any more, that supernovas and O - B associations are r-e-a-l-l-y messy housekeepers.) You would see decidedly pink hot spots (100,000 K hot) from H2 clouds heated by UV radiation and young stellar associations expelling excess gas. This mix of star-forming features was called a Cosmic Bubble Bath by Brand & Zealey as far back as 1975. The contribution of bubble-rim star formation to a galaxy can be considerable: last year Thompson & Urquhart estimated that between 14 and 30% of the massive stars in the Milky Way were formed this way.

For Northern observers who want to whet their skills for an attempt on the Fornax Dwarf and its globulars, you can practice on Coddington (see also the DSS images available on SkyView ). The Fornax/Coddington visual magnitudes are 8.8 vs 11.3 respectively, with surface brightnesses 17.0 vs 16.3. IC 2574 is a diminutive 15’x9’—which is approximately the same size I trace visually in the Fornax Dwarf using my 180mm Mak. The H2 bubbles in Coddington are roughly the same apparent size as Fornax’s globulars (dia.

1 arcmin), though surface brightness is 3 magnitudes fainter at 16.3 per sq.arcmin. If you can pick out the H2 regions in Coddington , you’ve left the starting blocks for the Fornax Dwarf.

There’s a good reason for naddering on about Coddington like this. As you noodle out its features, you are seeing a dwarf irregular (dIrr) galaxy that is being hewn by gravity and starburst into a dSph like the Phoenix Dwarf and Fornax. Like Fornax? Why? Because Coddington has a large halo of dark matter, as does Fornax. If Coddington didn’t have the dark matter, it would slowly convert into a dwarf elliptical.

A dSph’s star burst phase dissipates in a billion or three years, leaving the remaining gas enriched with heavier elements than before. A single stripped MW or M31 satellite galaxy produces a dozen or more blobs with masses of 10,000 to a million solar masses—plenty enough for later star forming episodes. Their sizes range from range two to 15 light years—dense enough to be trapped by the galaxy. Blobs like these have been identified as cold HI fragments around M31. These in turn are the raw material of the next star forming episode. The intricacies of all this affect even our modest, watery outpost in vastness. The oxygen so necessary to life is formed only in stars more massive than 8 times the sun, which in turn comprise only about 15% of the stars in a starburst. For red giants that end in a white dwarf, most of the oxygen remains trapped in the star. We’re lucky to have our wee dram.

In the Coddington the star-forming activity is in the galaxy’s overall disk and outer regions . In the Fornax Dwarf, it happened in the center. Why there? This question is peculiar to dSph galaxies, and it has kept astronomers happily devising mathematical models for decades. DSph core formation occurs opposite of the way stars form in the arms of spiral galaxies. In dSphs a massive shrinking gas cloud wrings out stars. The heavyweights go supernova. Their blast waves create bubbles of hot expanding gas, which compress more gas, which leads to bubble-rim star formation . . . on and on till the gas runs out or is blown out of the Dwarf’s Roche Limit where the galaxy’s gravitational potential ends. By then the gas is so tenuous (1 atom per cc) that the Dwarf’s gravity can’t pull it back. The usual outcome is a mass of well-enriched stars, clean of gas, and fated to a long uneventful life as a dim glow. Perversely, from the astrophysicist’s point of view, a number of ancient dwarfs e.g., Draco , Pegasus and Phoenix , are forming stars even now.

Consider the images presented on p.1025-1030 in this paper , which addresses how tidal disruption and ram pressure strip star-forming gas from dSphs that lack a dark matter halo. In “naked” dSph galaxies like Ursa Minor haplessly fated to pass near or through a parent galaxy’s disk, star formation is smothered by “local processes.” Tidal interaction with a massive galaxy initiates molecular cloud collapse. Some stars are ejected by the gravitational slingshot of close encounters. Supernova shock shells and high-energy particles associated with bubbles like those we see in Coddington induce turbulence, local-scale compression, and clouds of million-degree-plus ionized gas. These expand outward at high velocity. The ram pressure of passing through a gas-dense host galaxy disk, plus ultraviolet radiation from ultra-hot new O and B stars, heats and ejects any remaining unbound gas, creating magnetic fields that direct ion flow along their lines. Magnetic heating adds to already erosive thermal and UV heating, driving the gas even more speedily away from the galaxy’s core. Peace and quiet is never a sure thing in a budding dwarf galaxy.

None of this is headline material. M31’s dwarf galaxies appear to have an easier time of it. Unlike the other three dwarf galaxy classes, the bursts of star formation in the Fornax Dwarf are concentrated near its core. Each generation of new stars exhibits higher metallicity. But while the Milky Way halo contains extremely metal-poor stars, the dSph galaxies orbiting that halo average much higher metallicities* even after having lost 96 to 99 percent of it to the galactic medium. How can that be? It suggests that the gas that made the first generation of stars in the Fornax and other dSphs was somehow enriched with heavier elements . Dwarf spheroidals are a fruitful galaxy type for assessing the predictions of dark matter theory . (Further data here .)

* Note the word “average.” The Fornax, Sculptor, and Sextans dSphs certainly have their lion’s share of metal-poor first-generation stars. It’s the higher-than-average metallicities that have astrophysicists going through a lot of coffee. “Higher than average” makes for some interesting numbers. When we see the Fornax Dwarf in our eyepieces, we are seeing the aggregate luminosity of 61 million suns. Spectrograms reveal that this light combines 83,000 solar masses of iron, 29,000 solar masses of silicon, 12,000 solar masses of magnesium, and 1,900 solar masses of calcium (there really is a lot of starshine in every glass of milk.) Even so, the late bursts contribute only about 25% of the stars in the dSphs studied to date.

The star formation history of Fornax is more complex than its remote solitude and nearly polar retrograde (moving in the opposite direction as the galactic arms) orbit would have us assume. All dSphs contain ancient stars, but Fornax also has a considerable number of red giants, including carbon stars . The latter presence suggests a large intermediate-age population 3 to 5 billion years old, which identify a dSph’s star-forming epochs. Carbon stars are old stars in their late helium-burning phase when they are producing a high abundance of carbon and oxygen. Large convection cells in the interior dredge the scarbon and oxygen to the surface, where the temperature is low enough for carbon monoxide (CO) and other complex molecules to form. (One of those compounds is graphite think of that next time you sharpen your pencil.) The surface gases in turn are spewed into surrounding space as the star evolves toward the planetary nebula phase. The ¼ ly thick CO ring around TT Cygni (Credit: H. Olofsson (Stockholm Observatory) et al) illustrates how brief a single-compound emission era can be. Long exposure images of M57 and M76 show faint ejecta shells well outside the main ring or polar lobes—that’s up to half the original star out there, even before the planetary nebulosity itself forms. Moreover, carbon stars aren’t very big to start with: the originate at about 0.8 solar masses and shed about 40% of that. Carbon stars emit heavily in a handful of spiky spectral bands from 510 to 600 nm—hence the need for specially designed narrowband filters to weed out the carbon chatter from the Milky Way. High carbon content also distinguishes dSphs from their look-alike cousins, the massive globulars, which have none. In dwarf spheroidals, the higher the metallicity , the higher the CO content.

Middle age safely under its belt, Fornax had a late fling with a small gas cloud and produced a modest brood of young progeny—roughly 3 new stars every 10,000 years. Some are as young as 200 to 100 million years . One paper tentatively identifies a few stars as young as 10 million years. So many young stars in a not very youthful galaxy hints that a considerable amount of atomic and molecular hydrogen was available to a galaxy that should have used its gas reserves in the initial starburst. A second starburst becomes possible if the dwarf is in a remote orbit like Fornax, Sculptor, Carina, or Phoenix. Leftover gas from the initial burst was expelled in hot shells within the galaxy into cold space. There it cooled, lost the gas pressure that put it out there, and fell back into the core . The Phoenix Dwarf is a well-documented case of how this happens.

In the Fornax dSph, a re-accreted gas bubble gave Fornax enough core density to commence a second burst 6.7 billion years ago. More than half of this burst’s red giants display the color-magnitude signature of stars younger than four billion years. Then, between two and one billion years ago Fornax commenced a third burst—or rather, dribble—of star formation commenced that added a minuscule 1,500 stars from then until about 10 million years ago. Today it has only 5,000 solar masses of gas left. In the 61 million solar-mass Fornax galaxy, that’s a dust mote in a firefly swarm.

Of all the 24 dSphs orbiting the Milky Way, Fornax has experienced the most recent and clumpy star formation. More important, the young stars are preferentially located towards the center—a property common to dSphs. This is called a population gradient, and is a little like all the teen-agers living in the vibrant center of a town while the older folks live out in the sedate ‘burbs (an arrangement that probably relieves both groups mightily). This suggests that a reservoir of gas required for later star formation episodes was retained in the core of the dwarf’s dark-matter halo, which is nearly a thousand times more massive than the visible matter.

Where did the gas cloud come from, and why did it show up so late? Fornax is now in its third orbit of about 3.7 million years around the Milky Way’s plane and has three starbursts to show for it. The push-pull of tidal stirring and gas ram pressure while passing through the MW’s dense disk gas should have stripped away the Dwarf’s excess gas a long time ago. The Ursa Minor and Draco Dwarfs (the latter one of the smallest MW dwarfs at 600 ly in diameter) have no gas and no starburst history because they are close to the MW plane (as seen in the illustration at the top of this article) and have passed through many times. In fact, all dSph galaxies within the tidal radius of the MW (870,000 light years) have somewhat similar star formation histories and today’s absence of gas in their cores.

A tidal radius is the outer boundary of an object’s gravitational field, beyond which it can no longer pull any escaping objects back in. But Fornax, the Carina Dwarf (also a small one at 500 ly dia.), and a few others somehow managed to hold on to gas long enough for it to infall and produce a starburst. The starbursts seemed to get underway when the dwarfs reached their apogalacticon, the outermost point from the MW core on a dwarf’s orbit. Why there—and then? Amazingly, in this highly instrumented, computer-modeled astronomy world, to explain the star formation histories of Fornax and many other dSph galaxies, we need to go back to the early days: Kepler and his ellipses.

Dwarf galaxies in remote orbits retain significant amounts of their surrounding gas in what is called Roche Sphere clumping. The dwarfs form stars when they approach their perigalacticon passages, the point in their orbits when they are nearest the center of a host galaxy. After the dwarf’s initial starburst, leftover gas expelled by the heat of the new core stars expands outward in a more or less round shell. As it cools the gas loses internal heat and therefore pressure, so expansion slows to a stop. Gas that was pushed outside the perigalacticon Roche Sphere breaks up into individual clouds that have masses, sizes, and densities comparable to the HI clouds recently discovered around M31.

The above example was modeled for a perigalacticon of 100,000 light years and eccentricity of 0.6. (The Fornax perigalacticon is 460,000 ly and eccentricity 0.13.) The above dwarf galaxy is shown as a solid circle between two smaller dots which represent large H1 gas clouds. The circle represents the Roche Sphere surrounding the dwarf. The Sphere shrinks or expands according to its distance from the galactic center, shown here at apogalacticon, mid-way in its orbit, and at perigalacticon. The Roche Sphere in this example represents a 300 million solar-mass dwarf orbiting a 53 billion solar-mass main galaxy. Note how remote the dots are from the Roche Sphere at perigalacticon and how they gravitate toward the dwarf as it nears apogalacticon. The dwarf’s Roche Sphere shrinks or enlarges in inverse proportion to its distance from the main galaxy’s gravitational well. In actuality, most dwarfs have several clouds orbiting at random distances. Some were formed out of gas expelled in the Dwarf’s first star-forming phase. Others were there to begin with and only now have randomly drifted into the vicinity of the Dwarf. Such clouds can have up to a million solar masses. Though diffuse, they still have a large gravitational potential. As the dwarf orbits the main galaxy, two things happen:

(1) The gas clouds approach or recede from the dwarf’s center of mass in inverse response to its Roche Sphere size. At perigalacticon they are attracted toward the host galaxy and drift away from the dwarf. At perigalacticon the host galaxy’s gravitational well shrinks the Roche Sphere of the dwarf. At apogalacticon the dwarf’s gravity well prevails and pulls the clouds toward it. The Roche Sphere enlarges. If it envelopes the nearby gas clouds it pulls them in, initiating a starburst episode. In reality more than one cloud is usually involved.

(2) Following Kepler’s second law, the orbital velocity of the dwarf/gas cloud ensemble speeds up as they approach the perigalacticon. The leading cloud moves further ahead and the trailing cloud falls further behind the dwarf. But as the leading cloud enters the denser gases and strong radiation field of the main galaxy, the combined effect of this and Kepler’s second law is a coefficient of drag (technically its “radius of survival”) which slightly slows the cloud. As it swings past into the departure orbit, it has lost some of its velocity and distance from the dwarf. What happens to the trailing cloud is much more interesting. As the dwarf plows through the main galaxy’s gas at supersonic speeds, a bow shock sets up, rather like a jet airplane’s or high-velocity star . The shock wave affects the dwarf’s tenuous gas clouds much more than the dense dwarf. The bow shock also introduces turbulence in the dwarf’s wake, developing a low-pressure zone which makes it easier for the trailing cloud to catch up to the dwarf. Race car drivers do a version of this when they slipstream the car just ahead. Astronomers use the term “column of capture”. If that sounds ominous for the cloud, it is: its bachelor life out making the rounds is about to be over.

The above representation was made for a galaxy with different properties than Fornax—five times as much mass and an ellipticity of 0.6, while Fornax’s is a much flatter 0.13. It more closely resembles the properties of the Draco Dwarf. Although it isn’t an accurate representation for Fornax, it gets across the idea of how gas clouds interact with a changing Roche Sphere to produce multiple star births approximately tied to the orbital period of the dwarf. Three orbits and the Fornax Dwarf has finally run out of gas.

Well . . . not quite. Fornax is a bit kinkier than any simplified picture drawn so far. For one, the Fornax dSph passed through the 600,000 ly-long high-velocity cloud of the Magellanic stream , which stripped some of its gas some 200 million years ago (for visualization see this APOD image. Credit: David L. Nidever et al. , NRAO / AUI / NSF & A. Mellinger , LAB Survey , Parkes Obs. , Westerbork Obs. , Arecibo Obs. ). This appears to account for why the youngest Fornax star populations are not aligned with the main body and also are highly structured (see image on p.13 of this paper ) instead of being more evenly distributed like a globular cluster. Then too, evidence of ram stripping in the Pegasus Dwarf hints that there is a much larger, ultra-tenuous gas medium that perfuses the entire Local Group which can nonetheless exert gas stripping effects on the poorly bound halo gas of the Group’s dwarf and hobbit galaxies. There is also an unexplained arc-like concentration (upper right in the illustration below) which suggests that Fornax had an off-center accretion encounter with a gas cloud roughly two billion years ago. This event has not yet been satisfactorily explained.

Fornax’s main features . [Credit: Cosmology and Extragalactic Astrophysics ] The solid ellipses represent Fornax’s inner core and outer tidal radius of 6760 light years ( Mateo 1998 ). The core radius is approximately what we see when observing the Dwarf in our telescopes. The dashed ellipses represent the half-mass and half-light radii, which are not important to us here. The small circles represent Fornax’s five globular clusters they are labeled using the numbers Paul Hodge originally assigned to them back in 1961. The size of their circles are approximately equal to the their tidal radii ( Mackey & Gilmore 2003, also 2002 ). Note how Fornax 4 (aka Hodge 4) lies inside the core radius while the other four GCs lie in the zone between the core and the tidal radius. The arc in the upper rights is a mass concentration suspected to be an accretion from two billion-years ago ( Coleman et al. 2008 p.2).

Fornax contains the most recent star formation activity of any Local Group dSph. Most of its stars—including four of its five globulars—formed around 10 billion years ago. Fornax 4, the globular newbie, dates from only 6 billion years ago. Life was placid for awhile—the Dwarf churned out roughly three new stars every 10,000 years for over 6 billion years. The tracer for this is a slow, steady—the technical term is “monotonic”—increase in iron from the cores of supernovas and lighter elements from the shells of planetary nebulae. Between 7 and 6 billion years ago Fornax experienced a burst of star formation several times that of the archaic era—roughly a new star every hundred years. The formation rate quickly decreased. A smaller surge occurred 600 to 400 million years ago, this time at only twice the archaic formation rate. Finally 100 million years ago, a small blip of star formation occurred, creating only about 1500 solar masses total between then until formation ceased 10 million years ago. These 10 million-year-old stars are the youngest stars ever observed in a dSph.

How long will the five globulars stick around? Without dark matter they would have migrated to the center a long time ago. Calculations by Angus & Diaferio (see p.3) based on a fiducial (a point assumed as a fixed basis for comparison) origination point of 4550 light years of the center show that NGC 1049—the globular we see easiest from Earth—should decay to the galaxy center in 5 billion years. GCs 2 and 5 would hang around for 10 billion years, approximately their current age. GCs 1 (the smallest) and 4 (the youngest and next smallest) were born near the center and will hang on longer than another Hubble time, the current age of the universe. Neither NGC 1049 not GCs 2 and 5 show any signs of going anywhere anytime soon. Astronomers tend to step gingerly when it comes to calculations about visible matter when there’s a lot of dark matter around.

How do we know all this? In 2007-08 Fornax underwent some very high resolution photometry showed that its bursts of new star formation had two things in common: they occurred in the central 4,000 light years of the Dwarf’s core, and the most metals-rich stars were nearest the center.

Photometry of the Fornax Dwarf to mag 23. Region 1 (the core) contains approximately 28,000 sources. Region 2, the annulus between the core and 2x the core, shows another 41,000 stars Region 3 is between 2 and 3 times the core radius and has 31,000 stars Region 4 is beyond 3 times the core radius and has 51,000. The most densely packed feature is the red clump, which is made up of core-helium burning stars, and is essentially the young-to-intermediate age horizontal branch for the galaxy. For a full explanation of these C-M shapes and sizes, see Coleman and de Jong , Astrophysics 2008, Fig. 3, page 4.

The above C-M plots show how far we have come in our clarification of how dwarf galaxies work. The most important lesson we can draw from the image is that measurements of individual stellar positions and velocities give a more accurate picture of spectral structure when many different kinds of stellar components are involved. Fact dances on the tips of pinpoints. Fornax has given us some pinpoints we need.

Hence the overall picture of dwarf spheroidal galaxy evolution goes something like this: About 2 to 3 billion years after the Big Bang, immense blobs of dark matter and hydrogen clouds collapsed into a future dwarf’s halo. The gas condensed enough to form the first generation of stars. The heaviest stars quickly evolved to supernovas and blew chemistry and energy into the surrounding gas cloud, enriching the gas still left there. In some cases like the Ursa Minor and Draco dwarfs, the excess gas was lost to the Milky Way. But with Fornax, way out there in the tenuity of the galactic reach, the gravitational well of its dark matter halo was deep enough to collapse the cooled gas back toward the center of the dark halo. One orbit later another collapse ensued, this time only about a quarter as intense as the primordial burst, to form a new generation of chemically enriched stars. This became a cycle. Multiple generations of stars progressively increased the dwarf’s overall chemical abundance.

The exact processes of chemical enrichment and tidal evolution in star formation in low-mass galaxies like Fornax are still not fully understood. We can’t substantiate the most important data related to Fornax’s high mass-to-light ratio because it lies in the properties of the galaxies’ dark matter halos, and dark matter doesn’t divulge its data very candidly. One thing is certain, though: dSphs like Fornax are small in size but huge in implication.

To wrap this up and summarize the most important issues still to be studied:

• The least luminous galaxies can be the most interesting.
• The least luminous galaxies also seem to have the highest proportion of dark matter.
• This field will develop in proportion to the rate at which data sets grow in precision, and how accurately computer models can predict their behavior.
• Amassing structural and kinematic data is the journey, not the destination. The steppingstones include chemical abundances, star formation histories, stellar composition, internal structure, and the orbital stresses of gigalife wheeling around a huge, if distant, big brother.
• The outer stellar structures of dSphs carry valuable information about the gravitational competition between the Milky Way and its satellites.
• The standard CDM hypothesis requires baryon-physical processes in order to form constant-density cores in dwarf galaxy dark matter halos.(Further research on whether the internal mass distributions of galaxies correlate with baryon physics (e.g., luminosity, metallicity, star formation history, see Schulte-Ladbeck et al. here .

Glimpsed in the eyepiece of my modest little C6 Celestron, Fornax and its five globulars turned out to be truly a wonder to behold.

Many of the early citations in this article were to pallettes of arXiv , IOP , and Google Scholar papers and articles rather than individual studies.

The cosmological significance of dwarf galaxies is summarized in Regina Schulte-Ladbeck, Ulrich Hopp, Elias Brinks, and Andrey Kravtsov, Dwarf-Galaxy Cosmology , Hindawi Publishing Corporation, 2010.

Richard Powell’s An Atlas of the Universe is a handy visualization tool to get a sense of where, exactly, we are.

Walter, F., & Brinks, E. 1999, Astronomical Journal, 118:273-301, July 1999. above illustration on p.285.

And, on those nights when you’ve finally had it with Dancing With The Stars, try The Old Faithfuls:

## How well do we know the solar system's orbital parameters around the galactic center? Could the solar system be following a somewhat eccentric orbit?

If we do know the orbital parameters -- what are they? Is it a simple elliptical orbit or do gravitational interactions along the way make it significantly deviate from a simple Kepler orbit? If it is a simple Kepler orbit what are the parameters? Apoapsis, periapsis, inclination, etc.

I'm geniunely curious. I googled for it for about 20 minutes and it seems to me like all we really sort-of-know is our speed relative to the galactic center, and from that we are assuming the orbit is roughly circular -- but we do not know more than than that (and we don't even know our own speed with great precision).

Its not a simple orbit or exact circle.

The solar system moves like a wave and crosses the galactic plane around once every

35 million years. And the nearest and farthest distance from the core are not the same.

Thank you for taking the time to reply. This is an interesting read, although short, and no sources are linked. The one source he used (a website) is not coming up.

If we are oscillating up and down through the galactic plane, that isn' exactly a Kepler orbit. I get that the spacetime (gravitational) "landscape" we encounter as we move through the galaxy is complex and varied, thus modifying our motion (at least w.r.t the galactic plane).

The thing that is bothering me is:

I can't find any sources on how we know our assumed orbital parameters.

## Introduction

In terrestrial dynamics and hydrodynamics the body force is usually given by an external field (the earth gravitational field, an electric and/or magnetic field, etc.). The external earth gravitational field, being almost constant in intensity and direction on the length scales of practical interest for the simulated fluid, does not cause particular numerical problems. On the contrary, the situation is completely different when dealing with systems of astronomical nature, both when they are ‘particle’ systems (stellar clusters and galaxies, where the interaction potential is that of point masses) or gaseous systems (gas clouds, gas and dust distributions in galaxies, accreting flows onto compact objects, etc.), where both body- and surface-forces are present (self and external gravitational fields plus electro-magnetic fields give the body force, while pressure and viscosity contribute to the surface force). Of course, in astronomy also the two-phase (stars + gas) state is present (for instance, star forming regions or young open clusters). In all these cases self-gravity cannot be neglected, because it contributes significantly to the internal energy of the system and, so, it is an important engine of its evolution. As a matter of fact, astrophysical systems formation and evolution is governed by self-gravity for example, galaxies formed when the (centripetal) internal gravitation energy was able to overwhelm the (centrifugal) Hubble expansion. If we define the parameter α as the ratio of the self-gravitation energy to the energy given by the external gravitational field, we obtain α ≃ 10 −8 for a fluid system like the Garda lake in the north of Italy and α ≃ 10 −2 for both a typical globular cluster (GC) like M 13 in the gravitational field of our Galaxy and for a typical galaxy of a cluster of galaxies like Coma. This means that, even when the external field is not negligible, self-gravitation is a million times more relevant in astrophysics than in terrestrial physics. The necessary inclusion of self-gravity in the study of astrophysical systems causes great troubles, even in the regime of validity of Newtonian approximation. This because of the double divergence of the point-mass interaction potential, U i j ∝ 1 / r i j where r i j is the Euclidean distance between the ith and jth star in a system. As high energy physicists would say, the Newtonian potential shows indeed both an ultraviolet divergence ( U i j diverges for two particles approaching to zero distance) and an infrared divergence, this latter meaning that ∇ U i j never vanishes, even on the large scales. The absence of any preferred length- (and thus time-) scale in a self-gravitating system makes very difficult to simulate it, because one has to handle an enormous range of scales.

## Keywords

• APA
• Author
• BIBTEX
• Harvard
• Standard
• RIS
• Vancouver

In: Astronomical Journal , Vol. 126, No. 5 1775, 11.2003, p. 2346-2361.

Research output : Contribution to journal › Article › peer-review

T1 - Proper motions of dwarf spheroidal galaxies from Hubble space telescope imaging. II. Measurement for Carina

N2 - This article presents and discusses a measurement of the proper motion for the Carina dwarf spheroidal galaxy (dSph) from images in two distinct fields in the direction of Carina taken with the Hubble Space Telescope, at three epochs. Each field contains a confirmed quasi-stellar object that is the reference point for measuring the proper motion of the dSph. The consecutive epochs are 1-2 yr apart. The components of the measured proper motion for Carina, expressed in the equatorial coordinate system, are μ α = 22 ± 9 mas century -1 and μ δ = 15 ± 9 mas century -1. The quoted proper motion is a weighted mean of two independent measurements and has not been corrected for the motions of the Sun and of the local standard of rest. Given the proper motion and its uncertainty, integrating the family of possible orbits of Carina in a realistic gravitational potential for the Milky Way indicates that Carina is bound gravitationally to the Milky Way and is close to apogalacticon. The best estimate of, and the 95% confidence interval for, the apogalacticon of the orbit is 102 kpc and (102, 113) kpc, for the perigalacticon is 20 kpc and (3.0, 63) kpc, and for the orbital period is 1.4 Gyr and (1.3, 2.0) Gyr. Carina does not seem to be on a polar orbit. The best estimate of the inclination of the orbit with respect to the Galactic plane is 39°, but the 95% confidence interval is so wide, (23°, 102°), that it includes a polar orbit. We are unable to confirm or to rule out the membership of Carina in a "stream" of galaxies in the Galactic halo because the difference between the observed and predicted directions of the proper motion is 1.6 times the uncertainty of the difference. Carina must contain dark matter to have survived the tidal interaction with the Milky Way until the present. The triggering of star formation by perigalacticon passages and crossings of the Galactic disk do not explain the history of star formation in Carina.

AB - This article presents and discusses a measurement of the proper motion for the Carina dwarf spheroidal galaxy (dSph) from images in two distinct fields in the direction of Carina taken with the Hubble Space Telescope, at three epochs. Each field contains a confirmed quasi-stellar object that is the reference point for measuring the proper motion of the dSph. The consecutive epochs are 1-2 yr apart. The components of the measured proper motion for Carina, expressed in the equatorial coordinate system, are μ α = 22 ± 9 mas century -1 and μ δ = 15 ± 9 mas century -1. The quoted proper motion is a weighted mean of two independent measurements and has not been corrected for the motions of the Sun and of the local standard of rest. Given the proper motion and its uncertainty, integrating the family of possible orbits of Carina in a realistic gravitational potential for the Milky Way indicates that Carina is bound gravitationally to the Milky Way and is close to apogalacticon. The best estimate of, and the 95% confidence interval for, the apogalacticon of the orbit is 102 kpc and (102, 113) kpc, for the perigalacticon is 20 kpc and (3.0, 63) kpc, and for the orbital period is 1.4 Gyr and (1.3, 2.0) Gyr. Carina does not seem to be on a polar orbit. The best estimate of the inclination of the orbit with respect to the Galactic plane is 39°, but the 95% confidence interval is so wide, (23°, 102°), that it includes a polar orbit. We are unable to confirm or to rule out the membership of Carina in a "stream" of galaxies in the Galactic halo because the difference between the observed and predicted directions of the proper motion is 1.6 times the uncertainty of the difference. Carina must contain dark matter to have survived the tidal interaction with the Milky Way until the present. The triggering of star formation by perigalacticon passages and crossings of the Galactic disk do not explain the history of star formation in Carina.

## An Etymological Dictionary of Astronomy and AstrophysicsEnglish-French-Persian

1) General: The highest point or level.
2) Mathematics: The highest point of a geometric figure or solid relative to some line or plane.
3) Astro.: Solar apex. The point on the celestial sphere toward which the Sun and the solar system are moving relative to the stars in our vicinity.

L. apex "summit, peak, tip," probably related to apere "to fasten, fix," hence "the tip of anything".

Cakâd "summit of a mountain top, crown of the head, top of the forehead," from Mid.Pers. cakât "summit," cf. Skt. kakud-, kakuda- "peak, summit," L. cacumen "top, point," cumulus "heap."

The point in the orbit of a planet, or other object in the solar system, which is furthest from the Sun.

Aphelion, from L. aphelium, from Gk. → apo- + helios "sun," cognate with L. sol, Skt. surya, Av. hvar-, Mod.Pers. xor, hur, O.H.G. sunna, Ger. Sonne, E. sun PIE *sawel- "sun".

Apâhur, from Pers. prefix apâ, → apo-, + hur "sun."

The distance between the → Sun and an → object in orbit around it when they are at their farthest approach.

A lens designed so as to minimize both its → spherical aberration and → coma.

An → optical system that is able to produce an image essentially free from → spherical aberration and → coma. See also the → Abbe sine condition.

Freedom from spherical aberration and coma.

Aplanatism, from aplanatic, from a- "negation prefix" + Gk. plane "wandering," from planasthai "to wander" + -tic adjective-forming suffix.

Nâbirahi, from nâ- "negation prefix" + birah "a devious path a wanderer, who deviates, errs," + -i noun affix.

Prefix meaning "away from, off out of without," ap- before a vowel or h.

From Gk. apo "away from, from" cf. Av. apâ "away from, from."

Persian apâ- from Av. and O.Pers. apâ "away from, from". Compare with Skt. apa "away, off," L. ab- "from, away," Hittite appa, Gothic af-, Ger. ab-, E. of, off PIE *apo- "off, away."

The point in an orbit where the moving body lies furthest from the celestial body around which it turns.

Apoapsis, from → apo- + → apsis.

Apâhabâk, from apâ-, → apo-, + habâk, → apsis.

The point on an elliptic orbit at the greatest distance from the principal focus or center of attraction.

Apocenter, from → apo- + → center.

Apâmarkaz, apâkayân, from apâ-, → apo-, + markaz or kayân, → center.

Corrected for → spherical aberration at two wavelengths or colors and for → chromatic aberration at three wavelengths.

An optical system that is → apochromatic.

The capacity of an optical system to bring three widely separated wavelengths of light into a single focus.

To bring the wavelengths of the spatially separate colors to a common focus.

The point in a lunar orbit that is farthest from the center of the Lune. Also apolune.

Apocynthion, from → apo- "away from, off" + cynthion, from Gk. Cynthia "goddess of the Moon."

Apâmah, apâmâh, from apâ, → apo-, + mah, mâh, → Moon.

1) Generally, any process suppressing the secondary maxima of a diffraction pattern, such as the faint rings around the Airy disk of an optical image. This allows the telescope to resolve finer details.
2) Over a telescope aperture, the use of a screen that progressively cuts down, from the center to the edge of the aperture, the amount of light transmitted, in order to reduce diffraction.
3) A mathematical treatment carried out on data received from an interferometer before the Fourier transformation is calculated to obtain the spectrum.

Apodization from v. apodize, → a- "negation prefix" + pod from Gk. podos "foot" (compare with Pers. , see below) + -tion "noun forming suffix" literally "removing feet," i.e. suppressing the secondary maxima at the side of the Airy spot.

Pâzodâyi "removing feet," from "foot," Mid.Pers. pâd, pây , Av. pad-, Skt. pat, Gk. pos, gen. podos, L. pes, gen. pedis, PIE *pod-/*ped-. Zodâyi, n. from zodudan "to polish, clean," Mid.Pers. uzdâtan, Av. uzdâ-, from uz-, → ex-, + dâ- "make, create."

The point on an elliptic orbit at the greatest distance from the principal focus. Also knwon as → apocenter.

## Abstract

We present new CCD VI photometry of the distant globular cluster Pal 13. Fourier decomposition of the light curves of the three cluster member RRab stars lead to estimations of [Fe/H] = −1.65, and a distance of 23.67 ± 0.57 kpc. Light and colour near minimum phases for RRab stars leads to an estimate of E ( B − V ) = 0.104 ± 0.001. A V / ( V − I ) colour-magnitude diagram, built exclusively with likely star members, shows consistency with the above parameters and an age of 12 Gyrs. A search of variable stars in the field of view of our images revealed the variability of a red giant cluster member and of three probably non-member stars two RRab stars and one W Virginis star or CW. The GAIA proper motions of member stars in Pal 13 show a significant scatter, consistent with the scenario of the cluster being tidally stripped.

## یک ستاره در هفت آسمان

آیا خورشید به گرد چیزی می‌چرخد؟ تا جایی که می‌دانیم بله! خورشید، و کل سامانه‌ی خورشیدی به همراهش، کهکشان را دور می‌زنند. به هنگام این گردش، نسبت به مرکز کهکشان هم دور و نزدیک می‌شود. "اَپاکهکشان" (apogalacticon) یک گام برجسته در این سفر است: دورترین نقطه‌ی مدار کهکشانیِ خورشید نسبت به هسته‌ی کهکشان.

کهکشان راه شیری توده‌ای از بیش از ۱۰۰ میلیارد ستاره است که خورشید هم یکی از آنهاست. همه‌ی این ستارگان در اثر نیروی گرانش به درون ساختاری گِرده-مانند به پهنای ۱۰۰ هزار سال نوری (یا ۹۶۵ کوادریلیون کیلومتر!) و کلفتی حدود ۱۰۰۰ سال نوری کشیده شده‌اند. یک مدل بسیار خوب برای سنجش چنین ساختاری، یک سی‌دی یا دی‌وی‌دی است که پهنایش ۱۰۰ برابر کلفتی‌اش باشد.

همه‌ی ستارگانی که کهک شان‌مان را ساخته‌اند به گرد مرکز کهک شان می‌چرخند، درست مانند سیاره‌هایی که به گرد خور شید می‌چرخند. خور شید در هر ۲۵۰ میلیون سال یک بار با فاصله‌ی میانگین ۲۸۰۰۰ سال نوری از مرکز کهک شان، آن را دور می‌زند. این بدین معناست که خور شید در درازنای ۵ میلیارد سال زندگی‌ا ش، تاکنون حدود ۲۰ بار به گرد مرکز کهک شان چرخیده. ما اکنون داریم به سوی نقطه‌ای در صورت فلکی زانوزده (هرکول) می‌رویم، نقطه‌ای در آسمان به نام چکاد یا آماج خور شیدی. با داشتن سرعت حدود ۵۶۰ هزار مایل بر ساعت (۹۰۱ هزار کیلومتر بر ساعت)، خورشید -و همه‌ی سامانه‌ی خورشیدی- در هر دو هفته راهی به اندازه‌ی قطر مدار زمین را می‌پیمایند.

[بر پایه‌ی پژوهشی تازه که اینجا خواندید، فاصله‌ی کنونی خورشید از مرکز کهکشان ۲۸۵۰۰ سال نوری است.]

مدار خورشید هم درست مانند مدار زمین و دیگر سیاره‌ها، کاملا دایره‌ای نیست. مدارهای سیاره‌ها بیضی است؛ می‌توانید آن را مانند دایره‌ای بدانید که کش آمده. این بدان معنی‌ست که فاصله‌ی زمین تا خورشید ثابت نیست، بلکه در درازنای یک سال به اندازه‌ی چند درصد تغییر می‌کند. سیاره‌مان در آغاز دی ماه به نزدیک‌ترین فاصله تا خورشید می‌رسد و در آغاز تیر هم به دورترین فاصله.

بسیاری از اجرام مداری چنین رفتاری دارند. در چرخش یک جرم به گرد جرمی دیگر، نقطه‌ای هست که دو جرم در آن به هم نزدیک‌ترینند و نقطه‌ای هم هست که در آن از هم دورترینند. این دو نقطه به ترتیب پیراهَباک (periapsis) و اَپاهَباک (apoapsis) نامیده می‌شوند. گاهی اخترشناسان این دو واژه را برای اجرام ویژه‌ای تعریف می‌کنند. برای ماهواره‌هایی که در مدار زمینند، نزدیک‌ترین و دورترین نقطه‌ها پیرازَم (حضیض) و اَپازَم (اوج) نام دارند. برای مدار سیاره‌ها، جایی که سیاره به خورشید نزدیک‌ترین یا دورترین هستند به نام‌های پیراهور و اَپاهور (باز هم همان حضیض و اوج) خوانده می‌شوند.

و برای ستاره‌ای که به گرد مرکز کهکشان می‌چرخد، نزدیک‌ترین و دورترین نقطه به مرکز کهکشان به ترتیب پیراکهکشان (perigalacticon) و اَپاکهکشان (apogalacticon) نامیده می‌شوند.

چرخشِ چیزهایی به گرد چیزهای بزرگ‌تر موضوعی بسیار رایج در اخترشناسی است. ماه به گرد زمین می‌چرخد. زمین -و همه‌ی سیاره‌ها، سیارک‌ها، و دنباله‌دارها- به گرد خورشید می‌چرخند. همه‌ی سیاره‌ها به جز دو تا (ناهید و تیر) دارای ماه‌هایی هستند که به گردِشان می‌چرخند. ستارگان به گرد ستارگان دیگر می‌چرخند هر جایی از کیهان که نگاه کنیم مداری می‌بینیم. حتی کهکشان‌مان هم دارای چندین کهکشان کوچک‌ترِ ماهواره‌ایست. دو تا از بزرگ‌ترینِ این کهکشان‌های ماهواره‌ای (ابرهای ماژلانی) با چشم نامسلح هم در آسمان نیمکره‌ی جنوبی دیده می‌شوند!

مدار خورشید به گرد مرکز کهکشان در عمل به گونه‌ی چشمگیری پیچیده‌تر از مدار سیاره‌ها است. این مدار یک بیضی ساده نیست. خورشید در مسیر مداری‌اش به گرد مرکز راه شیری، در صفحه‌ی قرص کهکشان بالا و پایین هم می‌رود. می‌توانید این حرکت خورشید را مانند بالا و پایین رفتنِ اسب‌های یک چرخ و فلک (کاروسل) بدانید. ما تقریبا هر ۳۰ میلیون سال از صفحه‌ی کهکشان می‌گذریم [و از آن بالاتر یا پایین‌تر می‌رویم-م]. در زمان کنونی ما داریم رو به بالای صفحه‌ی کهکشان می‌رویم و تقریبا ۷۰ سال نوری بالاتر از آنیم.

مسیر سامانه‌ی خورشیدی پیامدهای جالبی برای اخترشناسانِ آینده دارد. از آنجایی که همه‌ی گاز و غبارهای میان‌ستاره‌ای در صفحه‌ی کهکشان جای دارند، دیدِ ما به مرکز کهکشان بسته است [این گازها و غبارها جلوی دیدمان را گرفته‌اند و مرکز کهکشان را نمی‌توانیم ببینیم-م]. تنها راه ما برای این که بتوانیم چیزی درباره‌ی قلب راه شیری بدانیم بهره‌گیری از تلسکوپ‌های فروسرخ بزرگی‌ست که می‌توانند در ابرهای غبار کیهانی نفوذ کنند. ولی تا حدود ۱۰ میلیون سال دیگر، به اندازه‌ی چشمگیری بالاتر از صفحه‌ی کهکشان، و بنابراین بالاتر از انبوه موادی که دیدمان را بسته‌اند خواهیم رفت. اگر در آن زمان هنوز انسانی زندگی کند، شاید بتواند بدون نیاز به رفتن از خانه، بیننده‌ی چشم‌اندازی خیره‌کننده از مرکز کهکشان باشد!

حرکت زمین در فضا پیچیده‌تر از آنست که می‌پندارید. ما می‌چرخیم (دور خودمان)، می‌گردیم (در مدار)، و می‌لنگیم (در صفحه‌ی کهکشان)- و کل سامانه‌ی خورشیدی دارد از فضاهای میان ستارگان می‌گذرد. اپاکهکشان یک نشانه در سفر کهکشانی ماست؛ نقطه‌ای‌ست که در آن، از مرکز کهکشان دورترین هستیم. سامانه‌ی خورشیدی با گذشتن از اپاکهکشان، دوباره آغاز به نزدیک شدن به مرکز کهکشان می‌کند، سرعت جابجایی‌اش در فضای میان‌ستاره‌ای بالا می‌رود، و یک سفر ۲۵۰ میلیون ساله‌ی دیگر را به گرد مرکز کهکشان می‌آغازد.

واژه‌نامه:
sun - solar system - apogalacticon - galactic orbit - core - Milky Way Galaxy - star - CD - DVD - galactic center - planet - Hercules constellation - solar apex - ellipse - periapsis - apoapsis - Earth - perigee - apogee - planetary orbit - perihelion - aphelion - perigalacticon - asteroid - comet - moon - Magellanic Clouds - galactic disk - plane - infrared