# Is there enough hydrogen left after a star dies so another star will have enough to light up?

A star consumes quite a lot of hydrogen in its life, and is pretty much "vacuuming" everything in its vicinity. After it dies (eventually by supernova which will spread all its composition over light years), is there enough hydrogen left in that area to light up a new star? And will that star be more short-lived compare to its predecessor?

Our sun is a 3rd or 4th generation star, so yes, there is enough hydrogen left over to create more stars.

We know this because our solar system is fairly rich in heavy elements, which means that there must have been at least 1, and probably 2 or 3 supernovas that created these heavier elements that created all of the rocky planets, asteroids, comets, etc.

It is doubtful that our sun will shed enough hydrogen to create yet another star. It's too small now.

Also, if you look at the pillars of creation, which is a nebula created by a supernova, you can see the early stages of star formation happening right now.

First, thanks to @LCD3 for leading me on the right path here. My original answer was inaccurate, and so I got rid of it.

A supernova occurs when a very massive star can no longer sustain enough nuclear fusion to combat the force of its own gravity pushing inwards on it. This happens after the star has gone through different stages of fusion. Typically, it begins with fusing hydrogen into helium. This is the type of fusion you have probably heard about the most because stars are largely hydrogen and helium. However, there are other fusion processes that are equally important when it comes to prolonging a star's life, which fuse together heavier elements.

A star begins by fusing hydrogen nuclei into helium nuclei deep in its core. This is how the star produces energy, and is indirectly responsible for the star shining. However, there is only so much of this fusion a star can undergo in its core. When the core hydrogen is depleted, the star beings fusing helium there. It continues hydrogen fusion in its outer layers, where there is still hydrogen. Eventually, the star runs out of helium in its core, and begins to fuse even heavier elements. Hydrogen fusion continues in the outermost layers, with helium fusion occurring in lower layers.

Unfortunately, the process can only go on for so long, and eventually the star can't fight gravity any longer. In very massive stars, this leads to a supernova, which flings off much of a star's mass into space. In all the cast-away matter, is there enough hydrogen left to form a new star? Well, there isn't nearly as much hydrogen as there was at the star's birth. In relatively low-mass supernova progenitors, there might not be enough hydrogen to form a new star. In very high-mass stars, however, there will still be a substantial amount left. Could this form a new star? Probably not for a long time, because the hydrogen will have been thrown off into space by the supernova, and the wouldn't be very dense. It wouldn't be easy for it to collapse into a gas cloud to form a protostar. I wouldn't rule this out for very high-mass stars, but in the remnants of many stars, there probably would not be enough hydrogen to form a new star.

I hope this helps.

Source for the layer explanation: http://www.astronomynotes.com/evolutn/s5.htm. Also, many thanks to @LCD3.

There are several misconceptions in your question.

First, a star does not vacuum everything in its vicinity. Rather it forms from a condensation in a gas cloud, which in turn collapses to a proto-star surrounded by a gas disc, which can contribute further material. Once formed in this way, a star typically does not acquire more gas (exceptions are symbiotic binary stars etc).

Second, a star with mass in excess of $sim8$M$_odot$ will (typically after a long time) suffer from a supernova, when most of its envelope is flung back into space. That gas is still mostly hydrogen, though enriched by 'metals' (non-primordial elements). However, the gas is hot and fast moving and hence not in a condition to form another star.

Third, the gas from the supernova will eventually mix with other gas and dissolve into the general pool of interstellar medium (ISM). Some of that may cool to form a molecular cloud (as gas cloud where molecular $H_2$ dominates), which in turn may become the site of new star formation.

We know that the Sun has been formed from enriched material, which is a mixture of primordial gas with the ejections of several supernovae.

## Is there enough hydrogen left after a star dies so another star will have enough to light up? - Astronomy

There is by definition five different types of ways a star can die off in our universe. The types are brown dwarfs, black dwarfs, white dwarfs, neutron stars and the elusive black holes. The decision on how the stars will die matters on one property, mass.

The first type, brown dwarf, is a relatively easy one to explain. A brown dwarf attains a mass that is no more then 75 times the mass of Jupiter (this is roughly 7 % of our sun). This extremely small mass star is so small it doesn't have the gravity needed to heat up its core to the necessary 3,000,000 K needed to start the fusion of hydrogen (although it does have a faint glow because its hot, it just doesn't burn). Thus it never truly lights up. This is the reason why most scientist refer to a brown dwarf as a failed star. It would also be needless to say that a brown dwarfs life is eternal unless it is sucked up by another mass like another brown dwarf.

The second type, black dwarfs, are just as easy to explain. Black dwarfs are results from small mass stars that range from 7% of the mass of the sun to about 75% the mass of our Sun. The star's gravity is enough for the star to ignite the fusion of hydrogen, but with small masses comes a slow fusion reaction. That is why small mass stars burn the longest (sometimes up to 200 million years!). When the fuel is done, the stars internal temperatures are not enough for the fusion of helium to proceed and the reaction halts. Then when the reaction halts, the force pushing on the star to push out ends and the only force left is gravity. This collapses the star which produces more heat that usually ignites the star to burn hydrogen a little bit longer. This process continues until all the fuel is gone, then with all of the hydrogen gone the star fizzles out and becomes a black dwarf.

White dwarfs and come from normal mass stars (This is the category in which our Sun falls into) and will usually live for 9 billion years. Normal stars are defined as a star that is able to pass the fusion of hydrogen to the fusion of helium. Although the process of helium has started it does not mean that the fusion of hydrogen stops. Hydrogen will continue burning as long as its presence remains. As the star burns helium it expands in size by the forces of the nuclear chain reaction in its center. Then if the star is large enough to burn the next element, it will. Near the end the star will start shedding its outer layers into space because the outer layer is so far from the center, the gravity is insufficient to keep it attached to the star. The gasses are then propelled by the solar wind at about 10 miles a second. Proceeding that, the star will project hotter gasses that speed away from the star at about 1000 miles a second that span 1,000 times the diameter of the stars solar system. Then like the second stage the star collapses due to the lack of fusion and the heat produced produces more fusion. When that process happens the star sheds massive amounts of mass into space until all that is left is the inner core. This finished mass is named a white dwarf.

Neutron stars and black holes come from the same source- large mass stars. The shear size of these large mass stars make the fusion reaction so fierce that they usually only last about 30,000,000 years. Although their life is like a normal mass star the large mass stars are so large and hot (3,000,000,000 K) that they are able to fuse all the elements up to Iron. One of the qualities of Iron is the fact that its molecular structure refuses to fuse. When the Iron is made the Iron continues to compress itself into a tiny ball which heats up the core further to 100,000,000,000 K. Now this iron centers mass determines the fate of the star. If the center is between 1.5 to 3.0 solar masses the Iron core will compress even further from its size of the planet earth to just a little smaller then Chico, CA across. This compression crushes the Iron to its atomic particle of neutrons. Then as if the star detests the fact that it was crushed into a little ball of neutrons and neutrinos the star explodes in what is known as a supernova. A supernova is so intense it is believed that it is equal in brightness as the entire milky way (our galaxy). After the collapse of the star all that is left is the center. In 1987 there was a supernova that was seen from earth. The star appeared like it was an average size star in the sky with the naked eye, but when it went supernova, it increased to the size of a baseball in the sky .

Now lets go back to the Iron center of the large mass star. If it is larger then 3.0 solar masses a very interesting thing happens. Everything happens just like a neutron star, but the gravity exerted passes the limit set that keeps to atoms from occupying the same spot in space. This object which occupies this spot in space is named a singularity because we don't really know what is truly there. This singularity and the effects it has on space as been given the name black hole, aptly named for the fact that spacetime is so warped that not even light emitted from the singularity (or from an unfortunate astronaut) could escape. This structure can be more easily thought of as a rip through the fabric of space. This structure was so complex that even Einstein wasn't even sure that the existence of such an object could even exist.

This picture demonstrates how more massive stars in smaller spaces warp the spacetime fabric.

## Is the Universe Dying?

Poor Universe, its demise announced right in it’s prime. At only 13.8 billion years old, when you peer across the multiverse it’s barely middle age. And yet, it sadly dwindles here in hospice.

Is it a Galactus infestation? The Unicronabetes? Time to let go, move on and find a new Universe, because this one is all but dead and gone and but a shell of its former self.

The news of imminent demise was recently broadcast in mid 2015. Based on research looking at the light coming from over 200,000 galaxies, they found that the galaxies are putting out half as much light as they were 2 billion years ago. So if our math is right, less light equals more death.

So tell it to me straight, Doctor Spaceman(SPAH-CHEM-AN), how long have we got? Astronomers have known for a long time that the Universe was much more active in the distant past, when everything was closer and denser, and better. Back then, more of it was the primordial hydrogen left over from the Big Bang, supplying galaxies for star formation. Currently, there are only 1 to 3 new stars formed in the Milky Way every year. Which is pretty slow by Milky Way standards.

Not even at the busiest time of star formation, our Sun formed 5 billion years ago. 5 billion years before that, just a short 4 billion after the Big Bang, star formation peaked out. There were 30 times more stars forming then, than we see today.

When stars were formed actually makes a difference. For example, the fact that it took so long for our Sun to form is a good thing. The heavier elements in the Solar System, really anything higher up the periodic table from hydrogen and helium, had to be formed inside other stars. Main sequence stars like our own Sun spew out heavier elements from their solar winds, while supernovae created the heaviest elements in a moment of catastrophic collapse. Astronomers are pretty sure we needed a few generations of stars to build up enough of the heavier elements that life depends on, and probably wouldn’t be here without it.

Even if life did form here on Earth billions of years ago, when the Universe was really cranking, it would wish it was never born. With 30 times as much star formation going on, there would be intense radiation blasting away from all these newly forming stars and their subsequent supernovae detonations. So be glad life formed when it did. Sometimes a little quiet is better.

So, how long has the Universe got? It appears that it’s not going to crash together in the future, it’s just going to keep on expanding, and expanding, forever and ever.

Our eyes would never see the Crab Nebula as this Hubble image shows it. Image credit: NASA, ESA, J. Hester and A. Loll (Arizona State University)

In a few billion years, star formation will be a fraction of what it is today. In a few trillion, only the longest lived, lowest mass red dwarfs will still be pushing out their feeble light. Then, one by one, galaxies will see their last star flicker and fade away into the darkness. Then there’ll only be dead stars and dead planets, cooling down to the background temperature of the Universe as their galaxies accelerate from one another into the expanding void.

Eventually everything will be black holes, or milling about waiting to be trapped in black holes. And these black holes themselves will take an incomprehensible mighty pile of years to evaporate away to nothing.

So yes, our Universe is dying. Just like in a cheery Sartre play, it started dying the moment it began its existence. According to astronomers, the Universe will never truly die. It’ll just reach a distant future when there’s so little usable energy, it’ll be mostly dead. Dead enough? Dead inside.

As Miracle Max knows, mostly dead is still slightly alive. Who knows what future civilizations will figure out in the googol years between then and now.

Too sad? Let’s wildly speculate on futuristic technologies advanced civilizations will use to outlast the heat death of the Universe or flat out cheat death and re-spark it into a whole new cycle of Universal renewal.

## Hubble finds supernova star system linked to potential 'zombie star'

Using NASA's Hubble Space Telescope, a team of astronomers has spotted a star system that could have left behind a "zombie star" after an unusually weak supernova explosion.

A supernova typically obliterates the exploding white dwarf, or dying star. On this occasion, scientists believe this faint supernova may have left behind a surviving portion of the dwarf star -- a sort of zombie star.

While examining Hubble images taken years before the stellar explosion, astronomers identified a blue companion star feeding energy to a white dwarf, a process that ignited a nuclear reaction and released this weak supernova blast. This supernova, Type Iax, is less common than its brighter cousin, Type Ia. Astronomers have identified more than 30 of these mini-supernovas that may leave behind a surviving white dwarf.

"Astronomers have been searching for decades for the star systems that produce Type Ia supernova explosions," said scientist Saurabh Jha of Rutgers University in Piscataway, New Jersey. "Type Ia's are important because they're used to measure vast cosmic distances and the expansion of the universe. But we have very few constraints on how any white dwarf explodes. The similarities between Type Iax's and normal Type Ia's make understanding Type Iax progenitors important, especially because no Type Ia progenitor has been conclusively identified. This discovery shows us one way that you can get a white dwarf explosion."

The team's results will appear in the Thursday, Aug. 7 edition of the journal Nature.

The weak supernova, dubbed SN 2012Z, resides in the host galaxy NGC 1309 which is 110 million light-years away. It was discovered in the Lick Observatory Supernova Search in January 2012. Luckily, Hubble's Advanced Camera for Surveys also observed NGC 1309 for several years prior the supernova outburst, which allowed scientists to compare before-and-after images.

Curtis McCully, a graduate student at Rutgers and lead author of the team's paper, sharpened the Hubble pre-explosion images and noticed a peculiar object near the location of the supernova.

"I was very surprised to see anything at the location of the supernova. We expected the progenitor system would be too faint to see, like in previous searches for normal Type Ia supernova progenitors. It is exciting when nature surprises us," McCully said.

After studying the object's colors and comparing with computer simulations of possible Type Iax progenitor systems, the team concluded they were seeing the light of a star that had lost its outer hydrogen envelope, revealing its helium core.

The team plans to use Hubble again in 2015 to observe the area, giving time for the supernova's light to dim enough to reveal any possible zombie star and helium companion to confirm their hypothesis.

"Back in 2009, when we were just starting to understand this class, we predicted these supernovae were produced by a white dwarf and helium star binary system," said team member Ryan Foley of the University of Illinois at Urbana-Champaign, who helped identify Type Iax supernovae as a new class. "There's still a little uncertainty in this study, but it is essentially validation of our claim."

One possible explanation for the unusual nature of SN 2012Z is that a game of seesaw ensued between the bigger and smaller of the star pair. The more massive star evolved more quickly to expand and dump its hydrogen and helium onto the smaller star. The rapidly evolving star became a white dwarf. The smaller star bulked up, grew larger and engulfed the white dwarf. The outer layers of this combined star were ejected, leaving behind the white dwarf and the helium core of the companion star. The white dwarf siphoned matter from the companion star until it became unstable and exploded as a mini-supernova, leaving behind a surviving zombie star.

Astronomers already have located the aftermath of another Type Iax supernova blast. Images were taken with Hubble in January 2013 of supernova 2008ha, located 69 million light-years away in the galaxy UGC 12682, in more than four years after it exploded. The images show an object in the area of the supernova that could be the zombie star or the companion. The findings will be published in The Astrophysical Journal.

"SN 2012Z is one of the more powerful Type Iax supernovae and SN 2008ha is one of the weakest of the class, showing that Type Iax systems are very diverse," explained Foley, lead author of the paper on SN 2008ha. "And perhaps that diversity is related to how each of these stars explodes. Because these supernovae don't destroy the white dwarf completely, we surmise that some of these explosions eject a little bit and some eject a whole lot."

The astronomers hope their new findings will spur the development of improved models for these white dwarf explosions and a more complete understanding of the relationship between Type Iax and normal Type Ia supernovae and their corresponding star systems.

## How long until the sun becomes a red giant?

The lifecycle of a star highlights the main phases that a star undergoes before it dies. The duration of time that a star takes to move through all the stages makes up the lifetime of the star.

Here, energy is the total amount of fuel in the sun that can be converted to light.

According to nuclear physics, 4 atoms of hydrogen fuse to form 1 atom of helium in the core of the sun.

The atomic mass of 4 hydrogen atoms=

The atomic mass of 1 atom of helium=4.002602

As you can see, the atomic mass of helium is less than the combined mass of 4 hydrogen atoms. There is a difference of approximately 0.7% between the masses. So where does that 0.7% of the mass go? This missing mass is the energy that the sun releases after nuclear fusion.

To find the potential energy left in the sun, we must use Einstein&rsquos formula of E=0.007mc 2 for the conversion of mass and energy. (Also, m=mass of the sun, c=speed of light)

Now, nuclear fusion occurs at very high temperatures, and not all parts of the sun are hot enough for such fusion to occur. The core, constituting approximately 10% of the mass of the sun, is the region where most of the nuclear fusion takes place.

This fact further modifies Einstein&rsquos formula as:

Energy remaining in the sun= 2

The rate at which the sun releases energy, also known as its luminosity, is approximately 3.8×10 26 Watts.

Hence, the time until the sun becomes a red giant is approximately 10 billion years.

## Practically Everything Leaves Something Behind

"Everyone must leave something behind when he dies, my grandfather said. A child or a book or a painting or a house or a wall built or a pair of shoes made. Or a garden planted. Something your hand touched some way so your soul has somewhere to go when you die, and when people look at that tree or that flower you planted, you're there." -Ray Bradbury

Today is Memorial Day here in the United States, where we honor all the soldiers who have fought and fallen for our country. The peace and prosperity that I have enjoyed my entire life is because of a price paid, many times over, mostly by people I've never met. So it goes with the Universe, too.

Image credit: NASA Ames Research Center artist's rendition of Kepler 9's planetary system.

Over here at Starts With A Bang, I can think of no better way to celebrate it than by telling the story of what gets left behind by the stars that live, die, and give life to the next generation of stars and planets in the Universe. Because they didn't start as stars, of course. They started as diffuse clouds of cold gas, long ago, that collapsed under their own gravity.

When those clouds collapse, and reach a certain density, star formation occurs. Out of this gas comes a whole variety of stars, dominated by hot, blue, massive stars, but full of the whole gamut of different young star types.

Image credit: NASA, ESA and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration.

Take a look at the core of this cluster, and look past the brightest, hot blue stars here in cluster NGC 3603, and you'll find something typical of all newly formed star clusters.

Yes, there are the hot, massive, ultra-luminous blue stars, but there are far more of the less massive, Sun-like stars among them, and an even greater number of dim, red stars in the mix. This cluster, a mere 20,000 light years away (in our own galaxy), is a typical example of a star-forming region in the Universe.

And every star in that image, just like every star ever formed in the Universe, will someday run out of fuel and die. But what will each star leave behind? Turns out, that's entirely dependent on how much mass your star has.

The lowest mass stars, the Red Dwarfs (or the M-stars, above), with 40% the mass of the Sun (or less), burn their fuel the most slowly. While our Sun will live for billions of years, M-stars can live for many trillions of years, burning coolly and slowly through their fuel, eventually turning all their hydrogen into helium, and then simply contracting down in their entirety to form a degenerate ball of atoms: a white dwarf star. More than 1,000,000 times denser than water and over 1,000 times denser than the center of our Sun, a white dwarf packs the mass of maybe a hundred thousand Earths into the volume of less than one.

The Sun and a white dwarf star modeled on IK Pegasi B, by wikimedia user RJHall.

That degenerate dwarf is all a Red Dwarf will leave behind. While M-stars are most stars -- about 75% by number -- they're also the least massive and arguably the least interesting: not a single red dwarf has been around long enough in our Universe to burn through all of its fuel. But the other types -- from K-class stars all the way up through the lower-mass B-stars -- will die in the same fashion our Sun will.

Unlike M-stars, these stars burn through their fuel more rapidly, so the hydrogen in the outer layers never gets a chance to burn. What's more, is that the helium in the core can fuse further into carbon, nitrogen, oxygen, and sometimes even heavier elements: all the way up to iron for a few of these stars. When they reach the end of their lives, the result is simply spectacular.

Image credit: NASA, ESA, HEIC, and The Hubble Heritage Team (STScI/AURA).

A planetary nebula, like the Cat's Eye Nebula shown here, consists of the outer layers of a star of one of these types, blown off in the violent death-throes of such a star, spanning only a few thousand years. The outer layers -- half the mass of a star, on average -- are made up of some 97% hydrogen, ideal for providing the fuel for future generations of stars, while the inner layers, made up of mostly Carbon and Oxygen, contract down to form a degenerate white dwarf.

These white dwarfs -- the eventual fate of maybe 799 out of every 800 stars in the Universe -- will someday be so common that they will outnumber all the living stars in the Universe. But not every star that lives will wind up as a white dwarf. These rarities, the one-in-800 stars that are massive enough, will die in the most spectacular of explosion of all: a type II supernova!

All stars born with more than about 4-5 times the mass of our Sun have enough fuel in them that they cannot form white dwarfs at their center the white dwarf itself would be too massive, and must continue towards an even denser state! Instead, most commonly, the atoms themselves, normally made of protons, neutrons, and electrons, wind up collapsing almost entirely into neutrons, forming a tiny, ultra-dense ball known as a neutron star.

Because stars rotate, these neutron stars wind up spinning incredibly rapidly, and hence with incredible magnetic fields trillions of times what we find at the surface of our Sun. As these stars rotate, up to nearly 1,000 times per second, they send out electromagnetic radiation along the star's north and south poles. The stars that point one of their poles at us appear to pulse, anywhere between about 1 and 1,000 times per second, and hence we call them pulsars.

Optical/X-ray Image composite credit: NASA/CXC/HST/ASU/J. Hester et al.

The oldest, fastest pulsars are some of the best natural clocks in the Universe you can look away for over a year and then look back, and you'll know whether the pulse you're looking at is a billion pulses into the future or a billion-and-one. Only recently have atomic clocks passed pulsars as the best clocks in the Universe. What's more, is that it isn't just the hydrogen-rich outer layers of a supernova that get blown off in a stellar death like this it's many of the heavier elements, too. In fact, type II supernovae are where practically all of the elements found on Earth originated!

But neutron stars aren't the fate of all type II supernovae, just most of them. The rarest of all star types -- the most massive O-stars -- can actually have three different fates, depending on their masses. If your star is too massive to produce a neutron star, because even neutron stars have a mass limit, you will get a black hole to go with your supernova instead!

Image credit: JILA / Andrew Hamilton / University of Colorado.

And this is true, unless your star -- like maybe only one out of a billion stars -- is more massive than 130 times our Sun is.

Because if you get more massive than that, your star can die in a very special type of explosion, known as a Pair-Instability Supernova, where a pressure drop at the core of a star causes runaway thermonuclear reactions, destroying the entire star and leaving absolutely nothing behind!

Image Credit: X-ray: NASA/CXC/SAO, Optical: NASA/HST, Radio: CSIRO/ATNF/ATCA.

But there is one more possible fate, for the star types so massive that it's thought we don't even have one like it in our galaxy! If a star is more than 250 times as massive as our Sun, the star undergoes tremendous amounts of photodisintegration, where the entire core of the star collapses into a black hole, and except for a couple of highly collimated jets, there isn't even a hint of an explosion -- much less a supernova -- at all.

Image Credit: NASA / SkyWorks Digital.

Rather, the parent star is destroyed and a very massive black hole is created in the most energetic single-star event known: a hypernova!

And so, in memory of all the stars that have ever lived, now you, too, know what it is that they've left behind. For those of you who enjoyed the children's version, consider this the one for adults: this is the beauty of the Universe inherent in the death and life of every star. Without all of this, we never would have gotten here, and billions of years in the future, the matter that makes us up will spread out among the cosmos, where it will create future generations of stars, planets, and possibly, once again, life.

## Merging neutron stars

When a star around eight to 15 times the mass of the Sun explodes, it too leaves a super dense remnant of its inner core behind: a neutron star.

And when the gravitational waves produced by the collision between two neutron stars was picked up last year, telescope observations of the event also helped fill in some of the blanks on the periodic table of the origin of the elements, Dr Tucker said.

While physicists pretty much knew that elements in the fourth line of the period table — from potassium to krypton — were created by exploding stars, "the latter part of the table has always been, 'Well, we think we know what happens there'.

"By seeing the neutron star merger, we saw it happen."

The light signature thrown off by the neutron star merger provided clues that the heaviest naturally occurring elements, including gold, platinum, radium, thorium and uranium may be produced in the cataclysmic collision known as a kilonova.

## Ep. 13: Where Do Stars Go When They Die?

We’ve celebrated the birth of new stars, but the stellar lifecycle doesn’t end there. Stars like our Sun will spend billions of years fusing together hydrogen and pumping out energy. And when the fuel runs out, their death is as interesting as their birth. This week Fraser and Pamela trace out this stellar evolution, and explain what the future holds for stars, large and small.

### Transcript: Where Do Stars Go When They Die?

Fraser Cain: All right, onto the show. Now, last week we talked about how stars form, and we wanted to continue the stellar life cycle this week and discuss what happens to stars after that, all the way to the end. Now, when we last met our hero, the sun, it had formed from a cloud of dust and gas and it cleared out its neighbourhood with powerful stellar winds. What next?

Dr. Pamela Gay: Well, once it clears out its neighborhood with powerful stellar winds, it happily sits there chewing up hydrogen atoms, and fuses them into helium. And it does this for billions and billions of years, to quote Carl Sagan. Now the thing is that the sun, while it seems to be our nice constant object in the sky, hanging out and doing the exact same thing day after day, year after year, itâ€™s not doing the same thing millennium after millennium. The sun is actually slowly heating up, and while it will keep doing the things itâ€™s doing for another five billion years or so, as itâ€™s doing it, itâ€™s going to heat up to the point that in just a few million years, our earth wonâ€™t be the happiest place to be living.

Fraser: How many million years?

Pamela: Letâ€™s think of this in terms of a clock. The sun is currently about 4.5 billion years old. So letâ€™s call that 4:30 am. Well, according to scientists Peter Ward and Donald Brownlee, at about 5 am, our one billion year old reign of animals and plants will come to an end. The planet will heat up to the point that itâ€™s no longer comfortable for life to survive. Thatâ€™s only about 500 million years away.

Fraser: Now why is the sun heating up like this? Thatâ€™s not fair!

Pamela: Life is rarely fair, however. As the core burns more and more hydrogen into helium, itâ€™s expanding, and the larger and larger core is producing more and more radiation, which is producing more and more heat, which is heating up our solar system more and more over time.

Fraser: I always thought that, you know, we would have this long period, billions and billions of years, that weâ€™d have nice comfortable temperatures, but thatâ€™s not true.

Pamela: Well, many elementary school textbooks lie to young children. Itâ€™s a good pasttime. And they say that the sun will be around for another 5 billion years, so donâ€™t worry about the fate of the planet. Well, yeah, itâ€™s going to be around as a main sequence star for 5 billion years, but all the problems seem to hide in the details, and the details here say the sun is going to be getting hotter. As it gets hotter, it heats up our planet until itâ€™s first too warm for life, and then by the time the sun is 8 billion years old (and itâ€™s only 3.5 billion years from now that that happens), our oceans are actually going to vaporize. The planet will be so hot that the oceans just canâ€™t stay liquid any longer. So itâ€™s all rather devastating. Now, our planet might be allowed to survive, itâ€™s just the life on the planet that wonâ€™t survive.

Fraser: Okay, so letâ€™s keep going.

Pamela: So, the sun bloats up eventually, and it runs out of hydrogen in the core. So, currently, our sun is supported by the radiation given off by the fusion of hydrogen and helium. Well, finally, about the end of these 12 billion years, our sun is going to run out of easily burned hydrogen in its core, and the coreâ€™s going to collapse back down. And as it begins to collapse, a shell of hydrogen around that core is going to ignite. So, the atmosphere collapses down, builds up pressure on that helium core in the center, and squishes a layer of material between that helium core and the outer atmosphere of the star. And the shell ignites. And when that shell ignites, our sun bloats up into a red giant star. And at this point, a few planets lose their lives Mercury, Venus, definitely toast. Most models now think the Earth will safely escape being consumed during this phase of the sunâ€™s life.

So now we have hydrogen burning in the shell, and we have the helium core. Well, as that hydrogen burns, itâ€™s producing more and more helium, and heavy things sink to the centre. Itâ€™s sort of like when you drop a rock into the water, it goes down to the bottom of the water. Well, when you create helium in that hydrogen shell, that heliumâ€™s heavier and it sinks to the centre of the star.

The helium core is getting bigger and bigger and bigger until eventually the helium core is so dense and is experiencing so much pressure that it ignites. And now weâ€™re burning helium in the center, we have a shell of hydrogen, and the star, it becomes whatâ€™s now called a horizontal branch star. This is the point in life when some stars actually become variable stars, they become RR Lyrae variable stars, which is one of my personal passions. Our sun probably isnâ€™t going to do that, its mass isnâ€™t quite right.

Fraser: What would happen in that situation, though?

Pamela: With pulsating variable stars, which are stars that arenâ€™t quite balanced, gravity tries to squish them down, and as they compress, they heat up. And the heat produces more light coming out of the centre. It accelerates the rate of fusion in the centre. And so the light goes pushing out, and the light pushes the star out past its equilibrium, and the star cools off. And then it compresses back down. And thereâ€™s a lot of complicated physics going on here, but basically, radiation and gravity are playing tug of war with the atmosphere of the star, and itâ€™s constantly going in and out over a period of just hours. Itâ€™s something thatâ€™s really cool to watch because you can see something that is, over just six hours, expanding and contracting like a beating heart.

Fraser: And now does it leave material behind with each expansion?

Pamela: Not that we know of. Stars are constantly giving off mass, but in this case itâ€™s literally like a beating heart. The atmosphere of the star is pulsing outwards and inwards, outwards and inwards, like a coherent object.

Fraser: What I wouldnâ€™t do to be able to see that up close.

Pamela: Oh, it would be absolutely amazing. RR Lyrae stars were one of my first loves, because Iâ€™ve been a geek for a long time.

Fraser: But thatâ€™s not our sun.

Pamela: Thatâ€™s not our sun.

Fraser: So what happens to our sun?

Pamela: Our sun just kind of hangs out burning helium in its core and hydrogen around it. But eventually it canâ€™t burn the helium any longer. It might start expanding out at this point, as it continues to now burn a shell of helium and a shell of hydrogen. And over time, itâ€™s not going to be able to have these fusion reactions going on any longer, either. And as the fusion reactions shut down, the starâ€™s atmosphere slowly drifts away. This is one of the sad parts of a starâ€™s life. As they get old, they canâ€™t hold themselves together anymore, and they puff off layers of their atmosphere. This is the old asymptotic giant branch star, and whatâ€™s left behind as the atmosphere is poofed away in these very sad, elderly behaviours is just the core of the star.

Fraser: Now what do those poofed off layers look like from earth? Can we see any of those?

Pamela: They get illuminated as beautiful nebula. So the core of the star is still sitting there. Itâ€™s really hot, and hot things radiate light. And that light is used to illuminate the puffed off layers of the atmosphere. This what we call a planetary nebula. As a star disbands into atmosphere flying away and core left behind, the core gets called a white dwarf star and that flying away atmosphereâ€™s called a planetary nebula. Over time, the atmosphere goes further and further away and white dwarf cools off more and more, and the entire system disappears.

Fraser: So whatâ€™s in the white dwarf star? Whatâ€™s left inside there?

Pamela: Itâ€™s whatever was left from the fusion process. You can end up with helium white dwarfs where you have just the helium core of a now dead star. You can have stars where that helium fused into carbon oxygen, and youâ€™re left with basically a diamond, a diamondâ€™s left behind. So you can end up with a diamond thatâ€™s roughly the size of the earth left behind by a star that had sufficient mass to get a carbon core.

Fraser: Okay so the, under the pressure of the star, the carbon just kind of gets organized into its most compact form.

Pamela: And that happens to be a crystal diamond.

Fraser: So you would have a diamond the size of the earth…

Pamela: A diamond the size of the earth.

Fraser: Sitting in space. So how long would that take?

Pamela: Well, so, the diamond itself forms over the millions of years that the starâ€™s a giant. Now, the white dwarf, the diamond starts off as this giant glowing hot thing that, while structurally similar to a diamond, isnâ€™t exactly something youâ€™d want to put on your hand even if your hand were big enough to support an earth-sized ring.

That white dwarf starts off at the temperature of around 100,000 degrees K. It does cool off very quickly initially, and in the first 100,000 million years, if you consider that quick, it cools 20,000 degrees. Then it takes another 800,000,000 years to go another 10 degrees cooler, and itâ€™s not for 4 to 5 billion years that the star finally cools down to the temperature of our sunâ€™s surface, which is 5,800 degrees K. So, it takes it a long time to get to the point where youâ€™d want to get anywhere near it. But you do have this giant glowing really hot diamond left behind.

Fraser: All right, so itâ€™s not all hopeless. We get some bling in the end of it.

Fraser: Okay, so letâ€™s go a little smaller. When we talked last week, we talked about a nebula of gas and dust and various knots forming, and some of the big knots were these massive stars, and weâ€™ll get to those in a bit, and then sort of medium stars were stars like our sun, but what about smaller ones?

Pamela: So, red dwarfs are objects that have more than 80 Jupiter masses. And they behave like normal stars they burn hydrogen in their cores. But, they burn this hydrogen, in some cases, for trillions of years. A star that is a tenth the mass of the sun will hang out burning hydrogen into helium for about 6 trillion years, which is way older than our 13.7 billion year old Universe. So any red dwarf that has ever formed is still doing its thing. So we have no observational evidence of what these things do next.

But as near as we can guess, because theyâ€™re such a low mass, they wonâ€™t be able to contract and burn the hydrogen shell or do anything with their helium at later points in their lives. So once they stop burning hydrogen in the core, theyâ€™re just sort of going to go out, and then thermally contract. So theyâ€™re going to hang out, gravitationally held together, and squish themselves, and squish themselves, as gravity makes the star smaller and smaller and smaller, until eventually they squish themselves into a very small white dwarf star. And so eventually theyâ€™ll organize themselves so that their structure is that same crystalline degenerate electron, which is a really complicated term which just means that the electrons are in their smallest possible way of hanging out together.

Fraser: So itâ€™s like over time, all of whatever material is in the star, once it runs out of fuel for fusion, it just organizes itself in the most compact form that it can, and then just cools down and thatâ€™s that.

Fraser: But weâ€™ve got to be looking at trillions of years before that happens.

Pamela: And we will not be there for that. But itâ€™s fun to think about whatâ€™s going to happen at the end of the Universe.

Fraser: So itâ€™s neat that no one has ever seen any of this, itâ€™s just purely theoretical at this point.

Pamela: Yeah, and also, itâ€™s a neat thing to think about, that any red dwarf ever formed is still alive. Imagine saying that any of one specific type of mouse that was ever created on the planet earth was still alive. Life doesnâ€™t do that, but stars do.

Fraser: So, letâ€™s go a little smaller, then. The stars that have enough hydrogen, or size, in them to burn as stars, you know, these red dwarf stars, what if they donâ€™t have enough hydrogen fuel? Letâ€™s get smaller.

Pamela: Itâ€™s not that they donâ€™t have enough fuel, itâ€™s that they donâ€™t have enough gravity to do anything useful with it. The next smallest objects are these brown dwarf stars. They range in size from about 13 times Jupiterâ€™s mass to somewhere around 75, 80, Jupiter masses — weâ€™re still working on figuring out theoretical limits. These stars, they have a special type of hydrogen in them, as all stars do, called deuterium.

Deuterium is hydrogen that has a neutron in the center as well as a proton. Most hydrogen is just a proton and an electron, if itâ€™s neutral. But sometimes you get this extra neutron thrown in there. And when you have this extra neutron thrown in, the deuterium, this hydrogen plus neutron, it burns easier. So, in objects that are 13 to 65 Jupiter masses, theyâ€™ll, for a short period of time, maybe about 10 million years, theyâ€™ll be able to fuse the deuterium. But once they stop fusing the deuterium, they really canâ€™t do anything else. Some of the bigger ones, those 65 to 80 Jupiter masses, they can also fuse some lithium. Lithium just eats itself naturally, if you look at it too hard in a star it burns up. But other than that they canâ€™t do anything.

Fraser: So how can we see them, then? Because weâ€™re turning them up all the time, now.

Pamela: Luckily, for the first million years that theyâ€™re around, as they collapse out of their parentâ€™s nebula, the molecular cloud that hey formed out of, they look like any other star except they have a lot of extra lithium in them, because lithium gets eaten very fast in other types of protostars. So, for the first million years, they look normal, theyâ€™re at high temperatures, and they burn the deuterium, theyâ€™re still thermally really hot, and then they cool off. And itâ€™s after they cool off that they sort of disappear, but initially, just thermal contraction heats them up enough that we can see them.

Fraser: So theyâ€™re just the particles of hydrogen crushing together and rubbing against each other, and thatâ€™s the heat, like all that remains from a fire.

Pamela: And any time you compress gas, the gas heats up. Itâ€™s sort of like if youâ€™re pumping air when you compact the air inside your bicycle pump, it heats up. Well, a collapsing star is basically the same process as the squished air inside your bicycle pump: as it gets squished together, it heats up. Heated gas gives off light, and so itâ€™s just the fact that itâ€™s contracting gravitationally that allows it to heat up, and itâ€™s the heating up that we see as light.

Fraser: Right, I guess thatâ€™s why we need the infrared telescopes like Spitzer to turn these up, because they see heat not light.

Pamela: And this is one of the reasons that the next generation space telescope, the James Webb telescope, is being built as an infrared observatory. Itâ€™s going allow us to more effectively look for things like brown dwarfs. Itâ€™s also going to allow us to look for things at the far distant edge of the Universe, but thatâ€™s a different problem. So itâ€™s in the infrared that weâ€™re finding all of these fascinating things that we never imagined when we confined ourselves to looking at the optically luminous universe.

Fraser: All right, letâ€™s go big, then. So, you know, we started out talking about a main sequence star like our sun, and we sort of looked at where things go, smaller from there, so letâ€™s look bigger. So what happens if we get stars that are bigger than our sun?

Pamela: Well, as you get bigger and bigger, things start to get messy. Really big stars are giving off so much light that that radiation pressure is blowing off the outer layers of the star. And, so, the star can star off huge, and then make itself small rather quickly. These things burn for millions of years. Our sun burns for billions. The big stars are sort of like the Ferraris: they are bright, flashy, go fast, die young, and eat fuel like nobodyâ€™s business.

Fraser: Thatâ€™s right, I remember last week we talked about how like, the earliest stars were mainly hydrogen, and could, you know, blow up or not necessarily have the same kinds of stellar winds as the ones that, these days, have lots of heavier elements. Thatâ€™s all brand new science, isnâ€™t it?

Pamela: Well, itâ€™s not brand new science but itâ€™s brand new stars that are doing it. Itâ€™s fascinating to look at these things. They are literally blowing themselves apart. Itâ€™s as though they are going so fast that they just canâ€™t hold themselves together any longer. Thereâ€™s so many analogies to Hollywood movie stars that I could go to, but I wonâ€™t. So they live hard, blow themselves apart, and if they blow themselves apart too much, when they finally die, they explode as supernovae but they leave behind a white dwarf.

So, you have to end up with a core larger than 1.4 solar masses, which is this magical number. If you have more than 1.4 times the mass of the sun, then left behind after supernova, that material will collapse into whatâ€™s called a neutron star. If you have less than 1.4 solar masses, you just end up with a white dwarf again.

Fraser: I see. So the star could start out quite large, but it could blow away so much material that it just, it canâ€™t make it down to a neutron star once itâ€™s done.

Pamela: Exactly. The cutoff, we think, is objects that, by the time they go supernova, which weâ€™ll talk more about next week, have more than 10 solar masses. They will end up, after the supernova, with a neutron star, and things that are below that end up with just another white dwarf.

Fraser: So, what is a neutron star?

Pamela: A neutron star is what happens when the gravitational power of an object is so great that it squishes the atoms to the point that the protons and the electrons go â€œoh no, Iâ€™m too big, I canâ€™t be here any longerâ€? and they merge together, give off energy, and form neutrons. So, the matter compacts itself down to its smallest possible form, which in this case is basically a crystalline structure of neutrons.

Fraser: And this is one of those, a teaspoon amount weighs, what is it, like, a teaspoon amount weighs as much as a city or something like that.

Pamela: Hereâ€™s a great way to look at it. A white dwarf that is just under the 1.4 magical solar masses level will be about the size of the earth. A neutron star thatâ€™s more than 1.4 times the size, the mass, of the sun, is only 10 kilometers across. You could pretty much dump one on New York City. And gravitationally it would destroy the earth, but thatâ€™s just how small they are. And then you have all that mass creating all of this gravitational attraction in a little tiny area.

Fraser: Now, do these megastars go through that same kind of red giant phase at the end?

Pamela: Because theyâ€™re spewing off mass and going through reactions so quickly, they donâ€™t have as dramatic a change as they go from main sequence to red giant. They do make the transitions in terms of the way they generate energy. They go from having hydrogen burning in their core to having helium burning in the core, and theyâ€™ll actually get to the point where theyâ€™re doing things like fusing oxygen, creating neon, they get to the point that they actually end up creating iron in their core. So you end up with an onion shell of layer upon layer of progressively heavier atoms as you go from the surface of the star down to the core of the star, where all these different layers are fusing higher and higher atoms.

One of the neat ramifications of this is that any element that you have on your body, in your body, in the room that youâ€™re sitting in as you listen to this show, it had to have come from these giant stars. Whatâ€™s even cooler is any element that you have that happens to be heavier than iron, it came from a supernova, but again, thatâ€™s for next week. So anything smaller than iron, and bigger than about carbon, nitrogen, and oxygen, was formed in these giant stars as they were madly spewing out light and throwing themselves apart as they had huge stellar winds spewing matter into space.

Fraser: And how long will they last?

Pamela: They last just millions of years. Some of them last as few as 10 million years. So, the little guys, they can last for 6 trillion years, and the biggest stars will only last for 10, 12 million years.

Fraser: And how big can they get?

Pamela: Well, weâ€™re still finding the limits. Occasionally people find objects that they claim are hundreds of times the sunâ€™s mass. Because these things spew their outer atmosphere into space so rapidly, we have to catch them right as they form to catch the moment when theyâ€™re absolutely their largest. These are very rare objects as well. Really big stars donâ€™t form in large numbers. But we do find things now and then that we thinks just might be hundreds of time the size of the sun.

Fraser: And theyâ€™ll die even faster?

Pamela: And theyâ€™ll die even faster.

Fraser: Well, I think thatâ€™s great. Weâ€™ve skirted around it, but next week – and weâ€™ve had a bunch of emails of this, â€œwhy wonâ€™t you talk about supernovae?â€? – we will talk about supernovae next week, and talk about the deaths of the really big stars. So, gotta wait until next week.

Pamela: Have an explosive time.

Fraser: All right, thanks

Pamela:. Weâ€™ll talk to you in a week.

Pamela: Okay, see you later, Fraser.

This transcript is not an exact match to the audio file. It has been edited for clarity. Transcription and editing by Beans Velocci.

## What is a Star?

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Look up in the night sky and you’ll see lots of stars. But what is a star? In a scientific sense, a star is ball of hydrogen and helium with enough mass that it can sustain nuclear fusion at its core. Our Sun is a star, of course, but they can come in different sizes and colors. So let’s learn what a star is.

75% of the matter in the Universe is hydrogen and 23% is helium these are the amounts left over from the Big Bang. These elements exist in large stable clouds of cold molecular gas. At some point a gravitational disturbance, like a supernova explosion or a galaxy collision will cause a cloud of gas to collapse, beginning the process of star formation.

As the gas collects together, it heats up. Conservation of momentum from the movement of all the particles in the cloud causes the whole cloud to begin spinning. Most of the mass collects in the center, but the rapid rotation of the cloud causes it to flatten out into a protoplanetary disk. It’s out of this disk that planets will eventually form, but that’s another story.

The protostar at the heart of the cloud heats up from the gravitational collapse of all the hydrogen and helium, and over the course of about 100,000 years, it gets hotter and hotter becoming a T Tauri star. Finally after about 100 million years of collapse, temperatures and pressures at its core become sufficient that nuclear fusion can ignite. From this point on, the object is a star.

Nuclear fusion is what defines a star, but they can vary in mass. And the different amounts of mass give a star its properties. The least massive star possible is about 75 times the mass of Jupiter. In other words, if you could find 74 more Jupiters and mash them together, you’d get a star. The most massive star possible is still an issue of scientific disagreement, but it’s thought to be about 150 times the mass of the Sun. More than that, and the star just can’t hold itself together.

The least massive stars are red dwarf stars, and will consume small amounts over tremendous periods of time. Astronomers have calculated that there are red dwarf stars that could live 10 trillion years. They put out a fraction of the energy released by the Sun. The largest supergiant stars, on the other hand, have very short lives. A star like Eta Carinae, with 150 times the mass of the Sun is emitting more than 1 million times as much energy as the Sun. It has probably only lasted a few million years and will soon detonate as a powerful supernova destroying itself completely.

Most stars are in the main sequence phase of their lives, where they’re doing hydrogen fusion in their cores. Once this hydrogen runs out, and only helium is left in the core, the stars have to burn something else. The largest stars can continue fusing heavier and heavier elements until they can’t sustain fusion any more. The smallest stars eject their outer layers and become white dwarf stars, while the more massive stars have much more violent ends, become neutron stars and even black holes.

We have written many articles about stars on Universe Today. Here’s an article about the difference between stars and planets, and here’s an article about how massive stars form.

We have recorded several episodes of Astronomy Cast about stars. Here are two that you might find helpful: Episode 12: Where Do Baby Stars Come From, and Episode 13: Where Do Stars Go When they Die?

## How to Observe the Carina Nebula

ThoughtCo / Carolyn Collins Petersen

Skygazers who venture to the southern reaches of the northern hemisphere and throughout the southern hemisphere can easily find the nebula in the heart of the constellation. It's very near the constellation Crux, also known as the Southern Cross. The Carina Nebula is a good naked-eye object ​and gets even better with a look through binoculars or a small telescope. Observers with good-sized telescopes can spend a lot of time exploring the Trumpler clusters, the Homunculus, Eta Carinae, and the Keyhole region at the heart of the nebula. The nebula is best viewed during the southern hemisphere summer and early autumn months (northern hemisphere winter and early spring).