Size comparison between the Sun and VY Canis Majoris, which once held the title of the largest known star in the Universe. Credit: Wikipedia Commons/Oona Räisänen
The biggest stars in the Universe are the red supergiant stars. And we’re talking really, really big. The largest known red supergiant is thought to be VY Canis Majoris, measuring about 1800 times the size of the Sun. Imagine if the Sun extended out to the orbit of Saturn. Let’s take a look at where red supergiant stars come from.
Red supergiants are similar to red giants. They form when a star runs out of hydrogen fuel in their core, begins collapsing, and then outer shells of hydrogen around the core get hot enough to begin fusion. While a red giant might form when a star with the mass of our Sun runs out of fuel, a red supergiant occurs when a star with more than 10 solar masses begins this phase.
The five largest known supergiants in the galaxy are red supergiants: VY Canis Majoris, Mu Cephei, KW Sagitarii, V354 Cephei, and KY Cygni. Each of these stars has a radius larger than 1500 times the size of the Sun. In comparison, regular red giant is only 200 to 800 times the size of the Sun.
Red supergiant stars don’t last long; typically only a few hundred thousand years, maybe up to a million. Within this period, the core of the red supergiant continues to fuse heavier and heavier elements. This process stops when iron builds up in the core of the star. Iron is the equivalent of ash when it comes to nuclear fusion. The process of fusing iron actually requires more energy than it releases.
At this point, many red supergiants will detonate as Type II supernovae.
Betelgeuse was the first star directly imaged -- besides our own Sun, of course. Image obtained by the Hubble Space Telescope. Credit: Andrea Dupree (Harvard-Smithsonian CfA), Ronald Gilliland (STScI), NASA and ESA
When star like our Sun reaches the end of its life, it enters one last phase, ballooning up to many times its original size. Astronomers call these objects red giant star, and you’ll want to learn more about them, since this is the future fate for the Sun. Don’t panic, we’ve got another 7 billion years or so before the Sun becomes a red giant star.
As you probably know, stars shine because they’re converting hydrogen into helium in their cores through a process called nuclear fusion. Our own Sun has been performing fusion at its core for 4.5 billion years, and will continue to do so for another 7 billions years, at least. The helium byproduct from this fusion reaction slowly builds up in the core of a star, and they have no way to get rid of it. Eventually, billions of year down the road, a star uses up the last of its hydrogen fuel.
Once a star exhausts this fuel source, it no longer has the outward light pressure to counteract the gravity pulling in on itself. And so, the star begins to collapse. Before the star can collapse too far, though, this contraction heats up a shell of hydrogen around the core of the star to the point that it can support nuclear fusion. The higher temperatures lead to increasing reaction rates, and the star’s energy output increases by a factor of 1000 to 1000x. This new extreme light pressure pushes out the star’s outer layers beginning its life as a red giant star.
A red giant will expand outward many times its original size. Our own Sun, for example will grow so large that it engulfs the orbits of Mercury, Venus and even Earth; although, it’s not certain if Earth will actually be destroyed when this happens.
The core of the star will become so hot and dense that the leftover helium fuel will no able to star fusing into heavier elements. Stars with the mass of our Sun will stop with helium, but more massive stars will keep going, fusing carbon and even heavier elements together.
Without any more fuel to burn, these stars will expel their outer layers and then contract down to become white dwarfs.
We have written many articles about stars here on Universe Today. Here’s an article about a planet surviving when its star became a red giant. And were you wondering if the Earth will survive when the Sun becomes a red giant?
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The color of a star comes from the temperature of its surface. The hottest stars are blue, cooler stars are white and yellow, and the coolest stars of all are red. Red stars come in one color, but many different shapes and sizes. Let’s take a look at the different kids of red stars that are out there in the Universe.
First, let’s talk about temperature. As I mentioned above, the color of a star comes from its surface temperature. A star that emits mostly red light will have a surface temperature of about 3,500 Kelvin. Just for comparison, the Sun’s surface temperature is about 6,000 Kelvin and emits yellow/white light.
Red Dwarf Stars
The first kind of red stars are red dwarfs. These are actually the most common stars in the Universe. The smallest red dwarf stars can be 7.5% the mass of our Sun, and be as large as about half the mass of the Sun. Even a star with this little mass has enough temperature and pressure at its core to carry out nuclear fusion. This is where atoms of hydrogen are fused into atoms of helium; this process releases lots and lots of heat.
Red dwarfs generate much less energy than a larger star like our Sun. In fact, a red dwarf emits 1/10,000th the energy. Even the largest red dwarf only has about 10% of the Sun’s luminosity. Red dwarfs use their hydrogen fuel slowly and so they last a very long time. It’s believed that a red dwarf star could survive for 10 trillion years.
Red Giant Stars
Stars like our Sun spend most of their lives as main sequence stars with a surface temperature that’s much hotter than a red star. But at the end of their lives, when they have used up all their hydrogen fuel, these medium-sized stars will puff out much bigger than their original size – these are red giants. When our Sun becomes a red giant, it will expand out to encompass the orbit of the Earth. After a few hundred million years, it will puff its outer layers and become a white dwarf star.
They start out as regular stars, but they grow to such an enormous size that their heat is spread out across a much larger surface area. This is why they appear very bright, but still have a red color.
Red Supergiant
The biggest stars in the Universe are the red supergiants. They’re not the most massive; Betelgeuse, for example, only has about 20 times the mass of the Sun. But the largest red supergiants can expand out to be more than 1500 times the size of the Sun. Imagine a star that engulfed the orbit of Saturn! Just like the red giants, red supergiants occur when a star has used up the hydrogen fuel in its core, and then expand out during their helium-burning phase. Red supergiants will have the mass to continue fusing elements in their cores, all the way up to iron.
Stars probably only exist as red supergiants for a few hundred thousand years; a million at most. At the end of this period, the star will have used up all the fuel it can in its core; most will detonate as Type II supernovae.
We have written many articles about stars here on Universe Today. Here’s an article about a planet surviving when its star became a red giant, and here’s a deathwatch on a red giant star.
HST image of the Red Rectangle. Photo: Van Winckel, M. Cohen, H. Bond, T. Gull, ESA, NASA
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Dust is everywhere in space, but the pervasive stuff is one thing astronomers know little about. Cosmic dust is also elusive, as it lasts only about 10,000 years, a brief period in the life of a star. “We not only do not know what the stuff is, but we do not know where it is made or how it gets into space,” said Donald York, a professor at the University of Chicago. But now York and a group of collaborators have observed a double-star system, HD 44179, that may be creating a fountain of dust. The discovery has wide-ranging implications, because dust is critical to scientific theories about how stars form.
The double star system sits within what astronomers call the Red Rectangle, a nebula full of gas and dust located approximately 2,300 light years from Earth.
One of the double stars is a post-asymptotic giant branch (post-AGB) star, a type of star astronomers regard as a likely source of dust. These stars, unlike the sun, have already burned all the hydrogen in their cores and have collapsed, burning a new fuel, helium.
During the transition between burning hydrogen and helium, which takes place over tens of thousands of years, these stars lose an outer layer of their atmosphere. Dust may form in this cooling layer, which radiation pressure coming from the star’s interior pushes out the dust away from the star, along with a fair amount of gas.
In double-star systems, a disk of material from the post-AGB star may form around the second smaller, more slowly evolving star. “When disks form in astronomy, they often form jets that blow part of the material out of the original system, distributing the material in space,” York explained. An artist’s rendition of the possible appearance of the double star system in the Red Rectangle nebula. Credit: Steve Lane
“If a cloud of gas and dust collapses under its own gravity, it immediately gets hotter and starts to evaporate,” York said. Something, possibly dust, must immediately cool the cloud to prevent it from reheating.
The giant star sitting in the Red Rectangle is among those that are far too hot to allow dust condensation within their atmospheres. And yet a giant ring of dusty gas encircles it.
Witt’s team made approximately 15 hours of observations on the double star over a seven-year period with the 3.5-meter telescope at Apache Point Observatory in New Mexico. “Our observations have shown that it is most likely the gravitational or tidal interaction between our Red Rectangle giant star and a close sun-like companion star that causes material to leave the envelope of the giant,” said collaborator Adolph Witt, from the University of Toledo.
Some of this material ends up in a disk of accumulating dust that surrounds that smaller companion star. Gradually, over a period of approximately 500 years, the material spirals into the smaller star.
Just before this happens, the smaller star ejects a small fraction of the accumulated matter in opposite directions via two gaseous jets, called “bipolar jets.”
Other quantities of the matter pulled from the envelope of the giant end up in a disk that skirts both stars, where it cools. “The heavy elements like iron, nickel, silicon, calcium and carbon condense out into solid grains, which we see as interstellar dust, once they leave the system,” Witt explained.
Cosmic dust production has eluded telescopic detection because it only lasts for perhaps 10,000 years—a brief period in the lifetime of a star. Astronomers have observed other objects similar to the Red Rectangle in Earth’s neighborhood of the Milky Way. This suggests that the process Witt’s team has observed is quite common when viewed over the lifetime of the galaxy.
“Processes very similar to what we are observing in the Red Rectangle nebula have happened maybe hundreds of millions of times since the formation of the Milky Way,” said Witt, who teamed up with longtime friends at Chicago for the study.
The team had set out to achieve a relatively modest goal: find the Red Rectangle’s source of far-ultraviolet radiation. The Red Rectangle displays several phenomena that require far-ultraviolet radiation as a power source. “The trouble is that the very luminous central star in the Red Rectangle is not hot enough to produce the required UV radiation,” Witt said, so he and his colleagues set out to find it.
It turned out neither star in the binary system is the source of the UV radiation, but rather the hot, inner region of the disk swirling around the secondary, which reaches temperatures near 20,000 degrees. Their observations, Witt said, “have been greatly more productive than we could have imagined in our wildest dreams.”
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White dwarf stars are the corpses of stars; what happens once they’ve used up all their fuel and lack the temperature and pressure to continue fusion in their core. A white dwarf will be the end for all the small and medium mass stars out there – 97% of the stars in the Universe will become white dwarfs. The most massive stars in the Universe will suffer far more violent ends as supernovae or neutron stars. Let’s take a look at white dwarf stars.
For the majority of its lifetime, a star is in the main sequence phase of life; it’s converting hydrogen into helium at its core, and producing a tremendous amount of energy. Eventually a star runs out of hydrogen fuel in its core and its fusion stops. The star starts to collapse, but then a new shell of hydrogen fuel gets going. This causes the outer envelope of the star to puff out into a red giant. If a star is large enough, it will even be able to begin helium burning in its core creating carbon.
Once this fuel runs out, though, that’s it. The star is completely out of fuel it can use, and so it puffs out its outer layers, revealing the hot carbon core; the leftover material from this last fusion reaction. The star is now a white dwarf. It starts out hot, the temperature that the star’s core was, but then it starts to cool down over time. Eventually, after billions and even trillions of years time, the white dwarf will cool down to the background temperature of the Universe.
A white dwarf star is roughly the same size as the Earth, but it’s extremely dense, compacting the core of the former star into a region only 10,000 km across. Their average density is about 1,000,000 times denser than the density of the Sun. A single sugar cube sized amount of white dwarf would weigh about 1 tonne.
White dwarfs can only be up to 1.4 solar masses. Beyond this point, the pressure exerted by the individual atoms can’t hold back the gravitational pressure pulling it together. The white dwarf would collapse down to a more compact object, like a neutron star or a black hole.
Red Dwarf star and planet. Artists impression (NASA)
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Our Sun is such a familiar sight in the sky that you might think stars like our Sun are common across the Universe. But the most common stars in the Universe are actually much smaller and less massive than the Sun. The Universe is filled with red dwarf stars.
Astronomers categorize a red dwarf as any star less than half the mass of the Sun, down to about 7.5% the mass of the Sun. Red dwarfs can’t get less massive than 0.075 times the mass of the Sun because then they’d be too small to sustain nuclear fusion in their cores.
Red dwarfs do everything at a slower rate. Since they’re a fraction of the mass of the Sun, red dwarfs generate as little as 1/10,000th the energy of the Sun. This means they consume their stores of hydrogen fuel at a fraction of the rate that a star like the Sun goes through. The largest known red dwarf has only 10% the luminosity of the Sun.
And red dwarfs have another advantage. Larger stars, like the Sun, have a core, surrounded by a radiative zone, surrounded by a convective zone. Energy can only pass from the core through the radiative zone by emission and absorption by particles in the zone. A single photon can take more than 100,000 years to make this journey. Outside the radiative zone is a star’s convective zone. In this region, columns of hot plasma carry the heat from the radiative zone up to the surface of the star.
Red dwarfs have no radiative zone, which means that the convective zone comes right down to the star’s core and carries away heat. It also mixes up the hydrogen fuel and carries away the helium by-product. Regular stars die when they use up just the hydrogen in their cores, while red dwarfs keep all their hydrogen mixed up and will only die when they’ve used up every last drop.
With such an efficient use of hydrogen, red dwarf stars with 10% the mass of the Sun are through to live 10 trillion years. Our own Sun will only last about 12 billion or so.
You might be interested to know that the closest star to Earth, Proxima Centauri, is a red dwarf star. Unfortunately, these stars are so small and dim that they can’t be seen without a telescope.
We have written many articles about stars on Universe Today. Here’s an article about how red dwarf stars might have tiny habitable zones. And here’s an article about how they destroy their dust disks.
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A star will live the majority of its live in the main sequence phase. This is where nuclear fusion of hydrogen into helium is happening in its core, and the light pressure of this energy balances out the gravitational collapse of the star. Before a star gets into the main sequence phase, though, it spends some time as a protostar – a baby star.
Stars form when vast clouds of cold molecular hydrogen and helium collapse under mutual gravity. This collapse could have been triggered by a galaxy collision, or the shockwave of a nearby supernova. As the cloud collapses, it breaks into fragments, each of which will eventually become a star of some size.
As the cloud contracts, it begins to increase in temperature. This comes from the conversion of gravitational energy into kinetic energy. The cloud continues to heat up, and the conservation of momentum of all the different particles causes the protostar to spin.
The collapse of the cloud happens fastest at its center, where the material is at the highest density and hottest temperature. Unfortunately these objects are shrouded in dust, and impossible to see with Earth-based observatories. They can be seen in infrared telescopes though, which can pierce through the veil of dust that shrouds them.
As the collapse continues, a disk of gas forms around the protostar, and bi-polar jets blast out from the top and bottom of the star. These produce spectacular shock waves in the clouds.
An object can be considered a protostar as long as material is still falling inward. After about 100,000 years or so, the protostar stops growing and the disk of material surrounding it is destroyed by radiation. It then becomes a T Tauri star, and is visible to Earth-based telescopes.
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Most of the stars in the Universe are in the main sequence stage of their lives, a point in their stellar evolution where they’re converting hydrogen into helium in their cores and releasing a tremendous amount of energy. Let’s example the main sequence phase of a star’s life and see what role it plays in a star’s evolution.
A star first forms out of a cold cloud of molecular hydrogen and helium. Mutual gravity pulls the stellar material together, and this gravitational energy heats it up. The star first goes through a protostar phase for about 100,000 years, and then a T Tauri phase, where it shines only with the energy released from its ongoing gravitational collapse. This second T Tauri phase lasts a further 100 million years or so.
Eventually temperatures and pressures in the core of the star are sufficient that it can ignite nuclear fusion, converting hydrogen atoms into helium. When this process gets going, a star is said to be in the main sequence phase of its life.
In a star like our Sun, the core accounts for about 20% of its radius. It’s inside this region where all the energy of the Sun is released. The energy released in the core must then travel slowly through a radiative zone, where photons of energy are absorbed and then re-emitted. Energy is then carried through a convective zone, where columns of hot plasma carry bubbles of heated gas to the surface of the Sun where it’s released. The material cools down and falls back down inside the Sun where it’s heated up again. This journey can take more than 100,000 years for a single photon to get from the core of a star out to its surface.
Over time, a star slowly uses up the supply of hydrogen in its core, and leftover helium builds up. But the main sequence phase can last a long time. Our Sun has already been in its main sequence for 4.5 billion years, and will probably last another 7.5 billion years before it runs out of fuel.
The smallest red dwarf stars can smolder in the main sequence phase for an estimated 10 trillion years! The largest supergiant stars might only last a few million. It all comes down to mass.
And mass defines how a star comes out of the main sequence phase of its life. For the smallest red dwarf stars, astronomers think they’ll just shut off once they’ve used up all their hydrogen, becoming white dwarfs. More massive stars, with up to 10 solar masses, will go through a red giant phase where they expand many times their original size before collapsing down to the white dwarf. And the most massive stars will just explode as supernovae.
We have written many articles about stars on Universe Today. Here’s an article about the entire life cycle of stars, and different types of stars.
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Look up into the sky and you’ll see the stars twinkling in different colors. Some are dull and red, while others are white and others look bright blue. So how do you get so many different star colors?
The color of a star depends on its surface temperature. Our Sun’s surface temperature is about 6,000 Kelvin. Although it looks yellow from here on Earth, the light of the Sun would actually look very white from space. This white light coming off of the Sun is because its temperature is 6,000 Kelvin. If the Sun were cooler, it would give off light more on the red end of the spectrum, and if the Sun were hotter, it would look more blue.
And that’s just what we see with other stars. The coolest stars in the Universe are the red dwarf stars. These are stars with just a fraction of the mass of our Sun (as low as 7.5% the mass of the Sun). They don’t burn as hot in their cores, and their surface temperature is about 3,500 Kelvin. The light released from their surface looks mostly red to our eyes (although there are different colors mixed up in there too, red is the majority).
This is also the color you see with red giant stars; solar-mass stars that ran out of hydrogen fuel and bloated up many times their original size. The luminosity of the star is spread out over the much larger surface area of the red giant and so they’re cooler,
On the opposite side of the spectrum are blue stars. These are stars with many times the mass of the Sun and so their surface temperatures are much hotter. Blue stars start out above 10,000 Kelvin but they can reach 40,000 Kelvin with the largest hypergiant stars.
We have written many articles about stars on Universe Today. Here’s an article about the biggest stars in the Universe.
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The core of a star is located inside the star in a region where the temperature and pressures are sufficient to ignite nuclear fusion, converting atoms of hydrogen into helium, and releasing a tremendous amount of heat.
The size of the core depends on the mass of the star. For example, our Sun measures 1,391,000 km across and is a fairly normal star. The core of the Sun makes up about 20% of the solar radius; about 278,000 km across. It’s within this region that temperatures reach 15,000,000 Kelvin and nuclear fusion can take place. Fusion doesn’t take place in any other part of the Sun.
As you know, stars can be larger or smaller than the Sun. Larger stars will have larger, hotter cores. The largest stars have cores of 18 million Kelvin, and inside this region hydrogen is fused into helium using a different process called the CNO cycle.
The least massive star capable of sustaining fusion in its core is about 7.5% the mass of the Sun. Below this size, temperatures are too low and you end up with a brown dwarf.
