# What's the correct distance to Betelgeuse?

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Reading [1], [2] and this smartphone app[3], I'm getting different answers to the question.

The smartphone app is stating 427ly. [1] states approx. 700 ly, and [2] states approx. 724ly.

Even if I take [1] as the correct value, why does [2] state 724ly and the app stating 427ly?

Thanks

[1] - https://en.wikipedia.org/wiki/Betelgeuse

[2] - https://earthsky.org/astronomy-essentials/how-far-is-betelgeuse

[3] - https://www.escapistgames.com/sc.html

Distance measurements for Betelgeuse are a bit of a mess. Solutions based on parallax would be ideal, but Betelgeuse has a rather large angular diameter at most wavelengths thanks to its extended envelope; optical and infrared observations usually fall in the 40-60 mas range (see Dolan et al. 2016 for a recent review), while radio observations show a disk of emission roughly twice that size (O'Gorman et al. 2017). The parallax is expected to be much smaller that the angular diameter, on the order of ~5 mas, and so it depends strongly on the choice of the center of emission.

One of the first decent parallax results was obtained by the Hipparcos satellite in 1997, whose astrometric measurements allowed comparatively precise measurements of location, parallax and proper motion for over 100,000 stars. Hipparcos measured a parallax for Betelgeuse of $$pi=7.63pm1.64$$ mas, corresponding to a distance of $$131pm30$$ pc$$^{dagger}$$. This is that 427 light-year number the app cited. The Hipparcos-only result was subsequently improved upon significantly by van Leeuwen 2007, who found $$pi=6.56pm0.83$$ mas, cutting the old uncertainty in half; this would correspond to a distance of 152 parsecs. If you're going to quote a Hipparcos result, this is the one to pick.

More recent results indicate that this value is likely too low. Combining Hipparocs data with multiple multifrequency radio measurements using the Very Large Array, ALMA and e-MERLIN (Harper et al. 2008, Harper et al. 2017) gives derived values of $$197pm45$$ pc and $$222^{+48}_{-34}$$ pc, with the former barely consistent with the purely optical results (the latter value converts to 724 light-years). These groups note that the Hipparcos stochastic astrometric solution required the addition of so-called "cosmic error" or "cosmic noise" terms to individual position measurements.

The papers above note that the photocenter at both and optical radio wavelengths does not coincide with the barycenter, and may change on timescales of months to years. Therefore, extended, long-term observations would be necessary to reduce any photospheric "jitter" or other variations which could lead to variations in emission and thus astrometric fitting. Harper et al. 2017 proposed joint ALMA and Expanded VLA/Jansky VLA observations and mm and sub-mm bands over a period several years, but also suggested that this would require "a Herculean effort" to achieve, for logistical reasons (how many telescope committees would be inclined to commit that much time up front?). Perhaps interest in Betelgeuse's recent luminosity dip could motivate this sort of observation.

$$^{dagger}$$Rob Jeffries makes the point that given the pretty terrible signal to noise ratios of many of these parallaxes (Hipparcos was in particular was bad, but none of the observations are amazing), it's not clear that you can really get good, meaningful uncertainties on the distance measurement from them via $$d=1/p$$. I agree; it's safe to say that the jury is still out on those, and any source that claims a distance and an error should give make that quite clear.

Current measurements give a distance "somewhere between 567 light years and 835 light years", with a "best guess" of 724 light years.

So wikipedia is being honest and saying "about 700". We really don't know any better.

Earthsky.org is giving the "best guess" value.

The app is probably using the distance measured by the Hippcaros space telescope. This value is generally considered to be an underestimate.

Another answer explains why we find it so hard to measure this distance: What will it finally take to accurately measure the distance to Betelgeuse?

Look at this article: https://arxiv.org/abs/1706.06020. Depends on studies distance varies and this is probably the reason of this discrepancy. For instance Hipparcos catalogue gives $$131^{+35}_{−23}$$~pc, assuming pc=3.26ly it gives $$sim$$427~ly. More details in attached article ;).

We are waiting for results from Gaia mission, because at first it made parallax of fainter stars and very bright, as Betelguse will come next, but I don't know when, maybe it was measured already, but I can't find any credible information. They wrote in https://www.aanda.org/articles/aa/pdf/2016/11/aa29272-16.pdf

"The 230 brightest stars in the sky (G < 3 mag, loosely referred to as very bright stars) receive a special treatment to ensure complete sky coverage at the bright end"

## Betelgeuse

Betelgeuse, Alpha Orionis, is the second brightest star in Orion constellation and the ninth brightest star in the sky. It is a supergiant star, distinctly red in colour, located at an approximate distance of 643 light years from Earth. It is an evolved star, one expected to explode as a supernova in a relatively near future.

Betelgeuse is a large, bright, massive star easily found in the sky in the winter months because it is part of a familiar pattern formed by the celestial Hunter. The red supergiant marks one of Orion‘s shoulders, while the hot, bright giant Bellatrix, Gamma Orionis, marks the other.

Betelgeuse is a variable star and, as its brightness changes, the star has been known to outshine the constellation’s brightest star, the blue supergiant Rigel, Beta Orionis.

Position of Betelgeuse in the constellation of Orion. Image: Akira Fujii

Alpha Orionis’ traditional name, Betelgeuse, originated in the Arabic phrase Yad al-Jauzā’, meaning “the shoulder of the central one” or “the hand of Orion,” referring to the star’s position in the constellation. The phrase al-Jauzā’ is the traditional Arabic name for Orion constellation. It can be loosely translated as “the central one,” referring to a mysterious woman.

The modern Arabic name for the constellation is al-Jabbār, meaning “the giant.” With the phrase Yad al-Jauzā’, Medieval translators misread the Arabic character for Y as B, which resulted in a mistranslation into “armpit of the giant.” The name Betelgeuse can be pronounced /ˈbiːtəldʒuːs/, /ˈbiːtəldʒuːz/, or /ˈbɛtəldʒuːz/. The original pronunciation is uncertain.

Betelgeuse and Bellatrix mark the two shoulders of Orion, the Hunter. Betelgeuse, marking the right shoulder, lies in the upper left corner of the constellation from our point view. It is one of the largest and most luminous stars known. Placed at the centre of our solar system, Alpha Orionis would extend beyond the asteroid belt, all the way to the orbit of Jupiter and possibly beyond.

Betelgeuse belongs to the spectral class M2Iab. The M refers to the star’s colour, red, and the ‘Iab’ suffix indicates that the star is an intermediate luminosity supergiant. It has an absolute magnitude of roughly -6.02.

The star’s mass is uncertain, but estimates range from 7.7 to 20 times that of the Sun. As a result of its high mass, Betelgeuse has evolved quickly and, even though it is less than than 10 million years old, it is nearing the end of its life cycle. The star is believed to have an average luminosity about 120,000 times that of the Sun.

Betelgeuse is classified as a semi-regular variable star. Its apparent magnitude varies from 0.2 to 1.2 over a period of about 400 days, which is the widest magnitude range for a first magnitude star. Because it varies in magnitude, Betelgeuse occasionally surpasses Procyon in Canis Minor in brightness and becomes the seventh brightest star in the sky. When at its brightest, Betelgeuse outshines Rigel, the brightest star in Orion, and becomes the sixth brightest star in the sky. At its faintest, it drops below Deneb, the brightest star in Cygnus, and shares the position of the 20th brightest star with Mimosa, Beta Crucis, located in Crux constellation and one of the stars that form the Southern Cross.

Betelgeuse is a pulsating red supergiant showing low-amplitude variations and periods of stable brightness. The star’s pulsations result in its absolute magnitude varying from -5.27 to -6.27. As its outer layers expand and contract, the surface increases and decreases, and the temperatures rises and falls. Betelgeuse pulsates because it has an unstable stellar atmosphere. When it contracts, it absorbs more of the energy that passes through it. As a result, its atmosphere heats up and expands. When the star expands, the atmosphere becomes less dense and cools down, which leads to another period of contraction.

This collage shows the Orion constellation in the sky (Betelgeuse is identified by the marker), a zoom towards Betelgeuse, and the sharpest ever image of this supergiant star, which was obtained with NACO on ESO’s Very Large Telescope. Image: ESO, P.Kervella, Digitized Sky Survey 2 and A. Fujii

There are several cycles to the star’s pulsations, with short-term variations of roughly 150 to 300 days, and longer, cyclic variations over a period of about 5.7 years.

As a result of the star’s variability, limb darkening, angular diameter that tends to vary with wavelength, and the star’s tendency to occasionally change shape, many of Betelgeuse’s properties are still uncertain. Because the star is rapidly losing mass, its surface is obscured by a large envelope of ejected material, approximately 250 times larger than the star itself, which makes measurements even more difficult.

In 1985, two close companions were discovered in orbit around Betelgeuse, but have not been confirmed by research in the years since.

Betelgeuse is easy to find in the sky because it is part of a prominent winter constellation, Orion the Hunter. From mid-September to mid-March, the star is visible from almost any location on the globe, with the exception of those south of 82°S.

For observers in the northern hemisphere, Betelgeuse rises in the east just after sunset in January. In March, it is due south early in the evening. By May, the star only briefly shows up on the western horizon after sunset. In the early summer, it travels behind the Sun and can’t be seen. A few months later, it can be seen appearing on the eastern horizon just before dawn. In June and July, Betelgeuse can’t be seen at night at all, only using a telescope in daylight.

## How far is Betelgeuse?

In the article, How far is Betelgeuse?, on earthsky.org, author Larry Sessions explains how astronomers measure the distance between stars that are far away. It is very difficult to directly measure the distances between faraway astronomical objects in the night sky. By using the concept of parallax, one would be able to measure the distance to nearby stars. As an example to understand what a parallax is, hold your arm directly out in front of you with your thumb pointed up. Pay attention to where it seems to be against the background. Close one eye while holding your arm in the same position. Now close the other eye while opening the first. Your thumb should have appeared to shift slightly to the side. That is because you have two eyes at two separate positions looking at one object independently from one another. The reason the object doesn’t appear to shift while looking at it with both eyes is because our brains automatically calculate the distances from which the view differs. Ancient astronomers thought that using this concept to measure the distance to stars would work in the same way. They were correct. So instead of using two eyes, they used two different locations of the Earth, two positions, each at opposite sides of the sun. Using the diameter of Earth’s orbit, one can observe an object and measure the distance easily.

Using this method, astronomers were able to correct their inaccurate original estimated distance to Betelgeuse from a parallax angle of 7.63 milliarcseconds and at a distance of 430 light-years to 5.07 milliarcseconds and at a distance of approximately 643 light-years away.

This article relates to our eleventh conceptual objective, “I can explain how astronomers study the properties of stars including: distance, size and mass.” In our Lecture Tutorials workbook, we looked at The Parsec section to help us understand how to use parallax. This is the apparent motion of nearby objects relative to distant objects. We were able to determine that the closer a star is, the larger the parallax angle. Likewise, the farther away a star is from Earth, the smaller the parallax angle. Another connection to class would be another Lecture Tutorials section, Parallax and Distance. Parallax angles are so small that they are measured in units of arcseconds. 1 arcsecond is 1/3600 of 1 degree. In this tutorial we were to able to follow a set of drawings of a starfield showing one star moving back and forth across the starfield with respect to the more distant stars. These drawings are taken at different times in the year to show the star gradually moving across the starfield. This star exhibits parallax.

I was interested in reading this article. I liked that the article gave a personal example of how to understand parallax and parallax angles. This concept is helpful in understanding how astronomers are able to measure distances between stars. Overall, I enjoyed reading this article.

## The difficult science of Betelgeuse’s distance

In August 1989, an Ariane 4 rocket carried the Hipparcos satellite of the European Space Agency (ESA) into orbit. During a four-year mission, Hipparcos made the most accurate set of astrometric observations ever made up to that moment. It measured with a high precision parallax angle and proper motion of almost 120,000 stars, operating outside the Earth’s atmosphere and, therefore, free from the distortions it produces on starlight. In 1997, the scientific results of the mission were finally published. It was then known that the parallax angle of Betelgeuse [1] measured by the satellite was equal to 7.63 mas (thousandths of an arc second). A distance of 131 parsecs corresponds to this angle, that is, 427 light-years (the value reported by some of the sources mentioned above).

However, assuming that that measurement of the parallax angle is correct, we must also consider the error margin reported in the Hipparcos catalog, equal to ±1.64 mas [2]. Making the calculations, we conclude that Betelgeuse could be at any distance between 108 and 167 parsecs, that is, between 352 and 544 light-years. A difference not quite negligible.

Ten years later, in 2007, Floor van Leeuwen, a Dutch astronomer, published a second version of the Hipparcos catalog, in which, using a different method of processing the raw data provided by the satellite, measurements of parallax angles were eventually obtained up to four times more precise than those contained in the first catalog. The parallax angle of Betelgeuse was now smaller — 6.55 mas — and the error margin was also smaller: ±0.83 mas. The new estimate of the distance obtained following this revision brings Betelgeuse to 153 parsecs from Earth (498 light-years). But, if we take into account, as it should be, even the error margin, we get that Betelgeuse could be at any distance between 135 and 175 parsecs, that is, between 442 and 570 light-years.

It seemed at this point that Hipparcos had put an end to the long-standing question of the distance of Betelgeuse. However, a new estimate appeared in a study published in The Astronomical Journal in April 2008 — about two hundred light-years more than the value measured by Hipparcos. The new research combined the data provided by ESA’s satellite with those obtained in radio waves through a series of measurements carried out in various eras with the Very Large Array (VLA) of the National Radio Astronomy Observatory (NRAO). The conclusions of Harper, Brown, and Guinan, the three authors of the study, were as follows:

Positions of Betelgeuse from high spatial resolution multi-wavelength VLA observations have lead to a reduction in the parallax, and an increased distance, as compared to that measured by Hipparcos. […] By combining the Hipparcos and VLA positional data, the different kinds of systematic errors may result in a more robust result. In a sense, the derived distance of 200 pc is a balance between the 131 pc Hipparcos distance and the radio which tends towards 250 pc. Future new radio positions should lead to an improved astrometric solution.

The final figure given by the three for the Betelgeuse distance was 197 ± 45 parsecs, that is, 642 light-years with an error margin of ±147 light-years. Based on this estimate, the red supergiant could be anywhere from 495 to 789 light-years away from Earth.

## How far is Betelgeuse? | Astronomy Essentials

An picture of Betelgeuse taken at sub-millimeter wavelengths by the Atacama Large Millimeter/submillimeter Array (ALMA). It reveals a bit of sizzling fuel barely protruding from the purple large star’s prolonged environment. Some of the information used to compute the most recent parallax for Betelgeuse got here from observations by ALMA. Image by way of ALMA.

Betelgeuse, the brilliant purple star within the constellation of Orion the Hunter, is ultimately stage of its stellar life. Astronomers have lengthy thought it’s going to sometime explode to change into a supernova. In late 2019 and early 2020, Betelgeuse generated lots of chatter on social media amongst astronomers. They questioned, considerably jokingly, if an explosion have been imminent as a result of the star has dimmed, unprecedentedly, by a noticeable quantity since late October 2019. As the information went mainstream, many individuals questioned how far Betelgeuse was from us and if an explosion might harm life on Earth. The excellent news is that if Betelgeuse explodes, it is shut sufficient to placed on a spectacular mild present, however far sufficient to not trigger us on Earth any hurt. To reply the gap query first, Betelgeuse is roughly 724 light-years away. But getting that reply, even for a comparatively close by star, is surprisingly tough.

It’s solely within the final 30 years, with using new applied sciences, that astronomers have obtained extra correct measurements for the gap to Betelgeuse and different close by stars. This advance started in 1989, when the European Space Agency (ESA) launched an area telescope known as Hipparcos, named after the well-known Greek astronomer Hipparchus. Over a number of years of observations, the Hipparcos house telescope supplied parallax and distance information for greater than 100,000 comparatively close by stars.

Those measurements turned the premise for a lot of the estimated distances to stars that you just see immediately.

The 2020 lunar calendars are right here! Order yours earlier than they’re gone. Nearly offered out!

When seen from two areas, there is a slight shift within the place of a close-by star with respect to distant background stars. For observations on Earth, taken six months aside, the separation between these two areas is the diameter of Earth’s orbit. The angle alpha is the parallax angle. Image by way of P.wormer / Wikimedia Commons.

The authentic Hipparcos information gave a parallax of seven.63 milliarcseconds for Betelgeuse that’s about one-millionth the width of the complete moon. Computations based mostly on that parallax yielded a distance of about 430 light-years.

However, Betelgeuse is what’s generally known as a variable star as a result of its brightness fluctuates with time (that stated, the current pleasure over Betelgeuse’s dimming is as a result of it’s the largest dip in brightness ever noticed). And therein started the problem in estimating Betelgeuse’s distance.

That’s as a result of subsequent research discovered an error within the strategies used for lowering the Hipparcos information for variable stars. An effort to appropriate these errors gave a parallax of 5.07 milliarcseconds, altering Betelgeuse’s estimated distance from 430 light-years to about 643 light-years, plus or minus 46 light-years.

But wait, there’s extra. In 2017, astronomers printed new calculations that additional refined Betelgeuse’s parallax to 4.51 milliiarcseconds. This new evaluation of information from Hipparcos additionally included observations from a number of ground-based radio telescopes. That positioned Betelgeuse at a distance of about 724 light-years, or, extra precisely, between 613 and 881 light-years when information uncertainties are included.

You may know that the European Space Agency’s Gaia astrometry mission has the aim of constructing a three-dimensional map of our Milky Way galaxy. At the time of its second information launch in April 2018, ESA stated Gaia’s information had already made potential:

… the richest star catalog thus far, together with high-precision measurements of almost 1.7 billion stars.

Yet Betelgeuse is not a kind of stars, and Gaia gained’t be used to discover a extra exact distance for Betelgeuse. The cause is that Betelgeuse is too shiny for the spacecraft’s sensors.

A map of Orion the Hunter, displaying the situation of Betelgeuse. Image by way of IAU / Sky & Telescope journal / Wikimedia Commons.

A phrase about parallax. Have you ever seen a close-by object from two totally different areas, and seen how its place modified with respect to distant landmarks? That is the impact known as parallax. To acquire a distance estimate, measurements of a close-by star’s place within the sky relative to distant background stars are obtained six months aside. During that point, Earth has traveled to the alternative aspect of its orbit, so the 2 areas are separated by the diameter of Earth’s orbit, about 186 million miles (300 million km). The distinction within the close by star’s relative place on the two areas permits astronomers to derive a parallax angle, and calculate a distance to the close by star.

Ancient Greek astronomers understood the idea of parallax, however they lacked the expertise to make very effective angular measurements on the sky. As a end result, all measurements of stellar parallax failed till German astronomer Friedrich Bessel succeeded in 1838. He used a telescope, and despite the fact that his two observing areas have been on reverse sides of Earth’s orbit, he was barely in a position to make out a tiny angular displacement. But it was sufficient to find out a distance of 11 light-years to a close-by star known as 61 Cygni.

From Bessel’s time till Hipparcos’ launch in 1989, only some thousand parallaxes had been decided. The course of was hindered by quite a few components together with the extraordinarily small angles concerned, imperfections within the devices, and maybe most of all, the murkiness of Earth’s personal environment. Observations from the Earth, even from very clear and darkish areas equivalent to deserts and mountaintops, are blurred by distortions within the environment.

Hipparcos, in acquiring observations from house beginning in 1989, pushed previous the restrictions imposed by Earth’s environment to get positional information of stars at unprecedented accuracy for that point. Astronomers are persevering with to refine these measurements with new improvements in devices and information evaluation, utilizing ground- and space-based observatories.

Bottom line: Measuring the gap to Betelgeuse has been significantly tough as a result of it is a variable star. Complex calculations based mostly on information from the Hipparcos house telescope and ground-based radio telescopes point out it is about 724 light-years away.

## How to pronounce Betelgeuse?

The star's name is derived from the Arabic يد الجوزاء Yad al-Jauzā', meaning "the Hand of al-Jauzā'", i.e. Orion, with mistransliteration into medieval Latin leading to the first character y being misread as a b.

So if we read it in latin it can be pronounced "Bee-tle-juz" or Bhe-tle-juz". Which pronunciation is considered correct by professional astronomers?

don't say it 2 more times though.

Dictionary.com has: [beet-l-jooz, bet-l-jœz], so I guess you win the thread.

"Bee-tle-juz" or Bhe-tle-juz" are both correct, the first being the most common.

It's pronounced just like the movie "Beetlejuice."

The character's name is actually shown several times during the movie as being spelled ➾telgeuse'.

The etymology of the name takes us from an Arabic source, to multiple badly transliterated Latin interpretations, to the present with multiple modern language adaptions, following some 20th century tweaks.

Anyone who says their pronunciation is right is right.

Anyone who says only their pronunciation is right is wrong.

As the most correct response here, I have no idea why your response isn't at the top of this thread.

There are three or four "frequently heard, generally accepted" pronunciations. But as you point out, the history and etymology of this name leaves it pretty wide open to interpretation.

Astronomers are even pretty sloppy with the -i and -ae pronunciations of the endings of constellations. To the point where sometimes using the "proper" Latin pronunciation may confuse some people.

## Don't panic! But Betelgeuse may be 25% closer to Earth than we previously thought

Hey, remember Betelgeuse? It caused quite a kerfuffle both among astronomers and normal people in late 2019 and early 2020 when the star's brightness dropped precipitously. There was much speculation in social media that it might explode, but astronomers knew this was incredibly unlikely. What they were more concerned with was why the star suddenly dimmed so dramatically: What was going on in the upper layers of this enormous star that could make such a huge difference in its light?

It seems pretty clear now that the red supergiant expelled an enormous cloud of dust, which blocks visible light, dimming it substantially. It's not clear why this happened and why on such a huge scale. Massive stars like Betelgeuse have complicated effects going on in their upper layers which can cause the star to physically pulsate, getting bigger and smaller over time. It's likely some other event happened in the star (perhaps a rising plume of hot gas) coupled with the normal pulsation, causing the creation of the dust that dimmed it.

A before-and-after set of images of Betelgeuse show how it’s changed from January 2019 (left) to December 2019 (right). Credit: ESO/M. Montargès et al.

This event, and the efforts to understand it, reminded astronomers that we really don't understand what's going inside this star very well, and in fact there are still basic facts about it we don't know! For example, its size, mass, age, and distance are all extremely difficult to determine.

However, a group of astronomers took a look at some old data of the star to measure its brightness changes, and by feeding them into some complicated models were able to get new estimates of these characteristics. Some of what they found is similar to older estimates, but their estimate of the size of the star has been revised downward quite a bit. The old estimates were it being 1.5 billion kilometers or so across, while the new one is now just over a billion. That's still ginormous, but a lot smaller than previous thinking

But if it's smaller, that means in turn it must be closer to us. They get a number of about 530 light years, 25% closer than previous estimates. That's significant!

Before going on, let me pause and say that if they're correct, and Betelgeuse is closer to us than previously thought, we are still in no danger if it goes supernova. At their new estimated distance it's still way too far away to hurt us if it explodes.

Oh, and the new research indicates it'll be a long, long time before it explodes anyway. Rest easy.

The astronomers wanted to investigate Betelgeuse using physical models of how gas flows inside the star, and how seismic waves travel through it, because these depend on the internal structure of the star (a little like how seismic waves traveling through the Earth tell geologists about the structure inside our planet). The change in brightness in the star over time depends on these factors as well.

Artwork showing the course of the dust eruption from Betelgeuse: A wave of hot, dense gas moves up and out from its deeper layers (panels 1 and 2), cools and heads away (panel 3), and how we saw it from Earth (panel 4). Credit: NASA, ESA, and E. Wheatley (STScI)

The astronomers needed better brightness observations of Betelgeuse to feed their models, so they turned to an unusual source: the Solar Mass Ejection Imager, a detector on the Department of Defense's Coriolis mission. This spacecraft was designed to observe the Sun and get a handle on its activity that can interfere with communications, as you might expect the DoD to be concerned about.

The instrument looked at the Sun, but also observed many bright stars including Betelgeuse. The astronomers reprocessed the data from that mission, providing measurements that filled a gap in observations in the late 2000s, giving them better data to use.

Betelgeuse is known to pulsate on multiple timescales, including one of several years' length and another of just over a year. Using the new data they found those periods to be 2365 days (6.5 years) and 416 days.

The models use that data, and also need things like the initial mass of the star (what mass it was born with red supergiants lose a lot of mass over time via a process like the solar wind but much more powerful), its age, and so on. Many of these numbers aren't pinned down, so the calculations are run with the input variables varied a little bit each time to see how the end results are changed. So if the initial mass of Betelgeuse is thought to be 18–21 times the Sun then the models are run with a mass of 18, then 18.1 (say), then 18.2, and so on. The results can then be analyzed statistically to see how robust they are.

A very deep exposure of the constellation of Orion, with Betelgeuse indicated by the arrow. Credit: Rogelio Bernal Andreo

And that's where the surprises came in. They find the size of the star dropped from something like 1100 times wider than the Sun to just ("just," ha!) 764 times wider. That's a big drop.

But there's more. We can measure Betelgeuse's apparent size on the sky — it's so enormous it's not just an unresolved dot, but can be seen as an actual disk. The size of that disk depends on the distance to the star and its actual, physical size. Now that they had the physical size, they could get the distance, and they found that it's about 530 light years away, when earlier numbers are more like 640.

That's a lot closer! And interesting, if true. Getting the distance to Betelgeuse is surprisingly hard. It's too bright for the Gaia observatory to observe, and other methods yield different values. 530 light years is consistent with old measurements from the Hipparcos satellite, for example, but disagrees with newer ones using radio observatories.

Orion rises in the east not long after sunset in December. Betelgeuse (just below and to the left of center) has faded dramatically recently, dropping in brightness to look more like the star Aldebaran in Taurus (top center). Credit: Phil Plait

Another interesting outcome of their calculations is that for all this to work, Betelgeuse has to be fusing helium into carbon in its core. Stars like the Sun fuse hydrogen into helium, and massive stars go through that phase in a few million years. After that they fuse helium, which lasts for about 100,000 years before moving onto to carbon fusion. After that the steps take shorter and shorter times, until the star tries to fuse iron, in which case it goes kaboom. Supernova.

So their models indicate it's still in the early stages of helium fusion, which in turn means it won't explode for a long, long time. Whether that makes you impatient or relieved is entirely up to you. I know it's too far to hurt us, so I'm in the former category. I want to see it explode.

So after all this, we still have an important question about this new work: Is it right?

Welllll. that's not easy to say. Their methods are interesting, but all of this is fraught with issues, some of which are really difficult to tease out. Any calculations using models like these have to make a lot of assumptions, and astronomers argue over the validity of those assumptions. Vociferously.

## Astro-Trivia: How do you pronounce "Betelgeuse"?

How do you pronounce "Betelgeuse"? When I was a teenager in the 1950s and 60s, it was always pronounced Bet-el-geeze. Then at some time, this "Beetle-juice" thing started, and that's how most people pronounce it today..

According to Wikipedia, "The star's name is derived from the Arabic يد الجوزاء Yad al-Jauzā', meaning "the hand of Orion". The Arabic letter for Y was misread as B by medieval translators, creating the initial B in Betelgeuse." So it should really be pronounced "Yettle-geeze"?

### #2 blb

Yep, pronounced Bet-el-geeze in the 60's and 70's, pre movie. After the movie Beetlejuice, every one pronounced it Beetle-juice. Now I do too.

### #3 catboat

الجوزاء al-jauza would be pronounced (classically) as al-jowza (rhymes with ‘WOW-za'). Some dialects, notably Egyptian arabic, pronounce ‘j’ as a hard 'g’ (the English word ‘jump' would become ‘gump’ -- as in Forrest G.)

How one handles the short initial vowel of definite article [al-] in a possessive compound depends on the grammatical case of the first term [yad]. As a fixed compound/name, the form you would mostly likely encounter would be yadu’l-jauza or yadu’l-gauza [Egyptian]. The short vowel ‘a’ of the word yad became a short ‘e’ when the word was taken up by speakers of other languages (probably by speakers of Turkish in this case, and then to European astronomers).

How people *should* pronounce the name of the star is another matter. Generally, we say whatever we think other people will think is correct.

### #4 Sarkikos

However most people pronounce a name or word in a particular language in a specific region is probably correct. Elite language academes can fight against this tendency, but it's often a lost cause. The problem is compounded when a word comes from a foreign language. It's even worse when the source was garbled to begin with, as is often the case with names in astronomy. A name going from Babylonian to Greek to Arabic to Latin to French to English is certain to sound different from the original source.

Probably best to pronounce words in common usage the way they are commonly pronounced. For instance, you would be more "correct" if you pronounce the Greek letter pi like "pee" instead of like "pie," and I've heard some folks say it that way. But then they might sound pretentious or silly or at any rate be misunderstood. Not good. The prime purpose of language is to convey meaning and be understood. Or at least it should be.

## What's the correct distance to Betelgeuse? - Astronomy

Unveiling the true face
of Betelgeuse
DR EMILY BALDWIN
ASTRONOMY NOW
Posted: July 29, 2009

Using state of the art imaging techniques, astronomers have revealed a vast plume of gas and gigantic bubbles boiling on the surface of Orion's supergiant star Betelgeuse. The new observation, the first of its kind, will provide important clues to help explain how these behemoths shed material at such an impressive rate.

State of the art observations reveal a vast plume of gas almost as large as our Solar System, and a gigantic bubble boiling on its surface. This artist's impression includes a scale in terms of the radius of Betelgeuse and the scale of the Solar System. Image: ESO/L. Calçada.

Betelgeuse rides on the shoulder of the constellation known as Orion the Hunter. At 1,000 times the size of our Sun it is one of the biggest stars known and also one of the most luminous, emitting more light than 100,000 Suns put together. But such mightiness comes at a cost, for Betelgeuse will meet its fate in a spectacular supernova explosion at an age of only a few million years.

Giant stars like Betelgeuse shed the equivalent mass of the Earth every year, but the mechanism of how they do so is poorly understood. "We know relatively well how much mass supergiants loose, and how it ends up in the interstellar medium as planetary nebulae," Pierre Kervella of the Paris Observatory tells Astronomy Now . "However, the mechanism of this mass loss is currently poorly understood, i.e. how physically the material escapes the gravitational field of the star."

Two teams of astronomers used ESO's Very Large Telescope to take steps closer to finding the answer. The first team used the adaptive optics instruments NACO, combined with the "lucky imaging" technique to obtain the sharpest view of the giant star ever obtained. Lucky imaging combines only the sharpest exposures to form an image much clearer than a single long exposure would provide. The resulting images have a resolution as fine as 37 milli-arcseconds, which is roughly the size of a tennis ball on the International Space Station, as seen from the ground.

Orion is easily recognisable by the three stars that make up the Hunter's belt, with red Betelgeuse riding on the Hunter's shoulder, indicated by the marker (left image). Zooming in on Betelgeuse (middle) and the sharpest ever image of this supergiant star (right) was obtained with NACO on ESO’s Very Large Telescope. Image: ESO, P.Kervella, Digitized Sky Survey 2 and A. Fujii.

"Thanks to these outstanding images, we have detected a large plume of gas extending into space from the surface of Betelgeuse," says team leader Kervella. The plume extends out to a distance at least six times the diameter of the star, corresponding to the distance between the Sun and Neptune. The images show that the whole outer shell of the star is not shedding material evenly in all directions. Kervella suggests two possible mechanisms for the asymmetry, associated with either large scale gas motions caused by heating, or because of the star's rotation.

"We think that convection at the surface, or the star's rotation can create sufficient momentum to eject the gas into space," he says. "The exact mechanism is however unknown for the moment. The convection is caused by vertical motion of material in the star. When it reaches the surface, it still has a significant vertical velocity, that can be sufficient to escape the star."

Kervella also suggests that despite Betelgeuse's slow rotation – it has a period of about 17 years – it might have a hot spot at its poles that would create additional pressure on the gas, forcing it into space. "Our observations are the first to establish a link between the surface of the star and its envelope," he says. "This is clearly a step towards a good comprehension of the mass loss mechanism for evolved stars."

A close look at Betelgeuse obtained with the NACO adaptive optics instrument on ESO’s Very Large Telescope. The image is based on data obtained in the near-infrared, through different filters. The field of view is about half an arcsecond wide, with north up and east left. Image: ESO and P. Kervella.

To probe Betelgeuse in even greater detail, Keiichi Ohnaka from the Max Planck Institute for Radio Astronomy in Germany and colleagues used the AMBER instrument on ESO’s Very Large Telescope Interferometer to obtain images equivalent to those taken with a 48-metre telescope. This provided even greater detail than the NACO images, equivalent to seeing a marble on the International Space Station from the ground.

"Our AMBER observations are the sharpest observations of any kind ever made of Betelgeuse," says Ohnaka. "Moreover, we detected how the gas is moving in different areas of Betelgeuse’s surface the first time this has been done for a star other than the Sun."Ohnaka's unrivalled observations show that the gas in Betelgeuse's atmosphere is bouncing vigorously up and down in bubbles that are as large as the supergiant star itself, and could be responsible for the ejection of the massive plume into space.

Kervella tells Astronomy Now that the behaviour seen at Betelgeuse could represent that at other red supergiant stars. "Many others have similar or even more extreme properties, so it is reasonable to expect similar properties in other stars," he says. "Betelgeuse has the advantage of being the nearest star of this kind."

Because of its proximity to Earth, when the star does go supernova it will be clearly visible with the unaided eye, even in daylight.

Betelgeuse also stole the show last month with the discovery that its core was shrinking, perhaps indicating the onset of a new phase of its evolution. See our news story and the August issue of Astronomy Now magazine for further information.

## The Spotty Surface of Betelgeuse

Today's APOD shows Betelgeuse. If I remember correctly, this was the first star resolved to a disk in the early 70s. It was the cover photo of one of the original Astronomy magazines at the time.

### #3 Neutrino?

Ok. That star is gnarly huge. I never knew that.

Jimmy- Since Betelgeuse is so massive, it will most likely TypeII core-collapse supernova and not form a planetary nebula. Thats only if it does not loose mass to the point where Fe fusion can not occur in its core and that its approximate mass is correct. From what I just read if the mass is a little less, it could become a rare neon-oxygen white dwarf though.

I wonder how old it really is though. What would a Type II SN look like from 600 light years away?

### #7 llanitedave

This leads directly to the three types of Supernova explosions:

Another view of Betelgeuse shows hot spots due to convection.

Assuming the distance to Betelgeuse is about 500 ly, and the diameter is 1 bn miles (they always say the Betelgeuse would extend beyong the orbit of Jupiter), the diameter of the disk in arcseconds would be 0.001. To get this detail the astronomers used interferometry from three different earth-based telescopes, separated by continental distances.

Using "Dawes limit," the size aperture required to resolve Betelgeuse to a disk would be about 4000 inches, or over 300 feet. The image looks to have at least ten times that resolution, which would call for an equivalent aperture of over 3000 ft. Over a half a mile. Synthetic aperture and interferometry certainly appears to be more cost effective

Imagine what we might discover with telescopes separated by solar system distances.

### #9 llanitedave

Imagine what we might discover with telescopes separated by solar system distances.

### #11 llanitedave

It will indeed pop "soon". Anytime between now and the next 50,000 years or so, I would imagine.

I don't think there's a formal upper size limit. It all depends on the interplay between the total mass of the star and the size and temperature of the core. Betelgeuse would be expected to have a massive, but very small and hot core containing multiple fusion shells. The outer shell is fusing hydrogen to helium, the next one in is fusing helium to carbon and oxygen, the one below that is fusing carbon and oxygen to, I think, elements such as nitrogen, sulphur, and silicon.

Each progressively heavier-element fusion requires higher temperatures and is less efficient, so lasts a shorter period of time. When it starts collecting iron in the core it will be pretty much over. I don't think there's any way to tell once it's reached that stage. The next stage, however, is the supernova blast when the iron begins to fuse and the whole structure immediately collapses in on itself. That will be pretty obvious.

Anyway, the higher the core temperature, the greater the thermal heating of the outer layers, so the more they expand. The more massive the star, the greater the gravitational pull preventing the expansion from blowing off the atmosphere entirely. Higher mass also means more core compaction and higher core temperatures.

In less massive stars, the outer layers do escape, and that loss of mass is one of the things that prevents the core from getting to the supernova stage.

Yes, stars that massive only last a few million years anyway, and indications are that this one is near the end. I wouldn't buy a ticket to see it, though. Probably not in our lifetimes.

But it would be most cool if it did

I did get an image of a supernova once, though. I happened to be imaging M51 the same time one blew. Didn't notice it, and only realized it because I went back and checked the photo after it was announced. M51 image here. Not a very good pic, but I put it up because it had the SN in it.

### #13 deSitter

It would suck if B went poof! No more Orion! Just Rion!

### #15 Neutrino?

A missing B would be pretty weird. then again we would have a crab nebula at 500 ly vs. 6000 ly. That would be pretty gnarly looking in a 1000 years.

There have been some formal workouts of an upper mass limit for stars but it is still up for debate as there is not a lot of supporting evidence. some stellar models work for 1000 Msol but we do not observed stars that larger. They either do not exist or there are none around because they are short lived.

Dave pretty much described the Eddington limit..which is just the need to maintain hydrostatic equilibrium.
From what I just learned in my astrophysics class I just took that was mostly stellar evolution.. the mass limit we see from observations is around 150 Msol. This comes from an area that is young and dense. can not remember where.

Dave also described basic core collapse. Each heavier element being fused releases more energy to neutrino cooling. Since neutrinos barely interact with the above matter. there is no support of the above material..to maintain hydrostatic equil. the fusion process speeds up.
Each shell of the "onion" burns more quickly..
C burning - 1000 years.
Ne - 1/2 yr
O - 1 yrs
Si- days
Ni- uh.
Fe = man down. Core collapse supernova. in which also those crazy neutrinos play a major role in.

Problem here is there are no observations - these stages are so short that they are completed faster than the thermal timescales of the star..the stellar surface does not know what is happening in the interior. some hydrodynamic models my prof presented showed C ignition occurs before thermal pulse-like double shell burning though. so maybe we will see that.

Also..I take my pizza thin and crispy!

Add: Tom that pic of the SN in M51 is freaking awesome. 23 million ly away holy *BLEEP*!