Astronomy Archives

February 5, 2009December 24, 2015 by Fraser Cain

Red Stars

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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.

Want more information on stars? Here’s Hubblesite’s News Releases about Stars, and more information from NASA’s imagine the Universe.

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?

References:
http://www.telescope.org/pparc/res8.html
http://en.wikipedia.org/wiki/Red_giant
http://en.wikipedia.org/wiki/Red_dwarf

February 5, 2009December 24, 2015 by Nancy Atkinson

Astronomers Find Cosmic Dust Fountain

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.”

Source: University of Chicago

February 4, 2009December 24, 2015 by Fraser Cain

White Dwarf Stars

Not a black dwarf ... yet (white dwarf Sirius B)

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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.

We have written many articles about stars on Universe Today. Here’s an article about a new type of white dwarf star detected. And here’s an article about a missing white dwarf.

Want more information on stars? Here’s Hubblesite’s News Releases about Stars, and more information from NASA’s imagine the Universe.

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?

References:
NASA
Wikipedia
Windows to Universe

February 4, 2009December 24, 2015 by Fraser Cain

Red Dwarf Stars

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.

Want more information on stars? Here’s Hubblesite’s News Releases about Stars, and more information from NASA’s imagine the Universe.

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?

References:
http://en.wikipedia.org/wiki/Red_dwarf
http://adsabs.harvard.edu/full/1953ApJ…118..529O

February 4, 2009December 24, 2015 by Fraser Cain

Protostar

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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.

We have written many articles about stars on Universe Today. Here’s one article about protostars, and here’s another.

Want more information on stars? Here’s Hubblesite’s News Releases about Stars, and more information from NASA’s imagine the Universe.

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?

February 4, 2009April 26, 2016 by Fraser Cain

Star Main Sequence

Stellar Evolution. Image credit: Chandra

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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.

Want more information on stars? Here’s Hubblesite’s News Releases about Stars, and more information from NASA’s imagine the Universe.

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?

References:
NASA
http://burro.astr.cwru.edu/stu/stars_lifedeath.html

February 4, 2009December 24, 2015 by Fraser Cain

Color of Stars

Star classifications. Image credit: Kieff

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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.

Want more information on stars? Here’s Hubblesite’s News Releases about Stars, and more information from NASA’s imagine the Universe.

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?

February 4, 2009December 24, 2015 by Fraser Cain

Core of a Star

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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.

We have written many articles about stars on Universe Today. Here’s a more detailed article about the core of the Sun, and here’s a nice diagram of the Sun.

Want more information on stars? Here’s Hubblesite’s News Releases about Stars, and more information from NASA’s imagine the Universe.

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?

February 4, 2009October 10, 2016 by Fraser Cain

What is the Life Cycle of Stars?

Much like any living being, stars go through a natural cycle. This begins with birth, extends through a lifespan characterized by change and growth, and ends in death. Of course, we’re talking about stars here, and the way they’re born, live and die is completely different from any life form we are familiar with.

For one, the timescales are entirely different, lasting on the order of billions of years. Also, the changes they go through during their lifespan are entirely different too. And when they die, the consequences are, shall we say, much more visible? Let’s take a look at the life cycle of stars.

Molecular Clouds:

Stars start out as vast clouds of cold molecular gas. The gas cloud could be floating in a galaxy for millions of years, but then some event causes it to begin collapsing down under its own gravity. For example when galaxies collide, regions of cold gas are given the kick they need to start collapsing. It can also happen when the shockwave of a nearby supernova passes through a region.

As it collapses, the interstellar cloud breaks up into smaller and smaller pieces, and each one of these collapses inward on itself. Each of these pieces will become a star. As the cloud collapses, the gravitational energy causes it to heat up, and the conservation of momentum from all the individual particles causes it to spin.

Protostar:

As the stellar material pulls tighter and tighter together, it heats up pushing against further gravitational collapse. At this point, the object is known as a protostar. Surrounding the protostar is a circumstellar disk of additional material. Some of this continues to spiral inward, layering additional mass onto the star. The rest will remain in place and eventually form a planetary system.

Depending on the stars mass, the protostar phase of stellar evolution will be short compared to its overall life span. For those that have one Solar Mass (i.e the same mass as our Sun), it lasts about 1000,000 years.

T Tauri Star:

A T Tauri star begins when material stops falling onto the protostar, and it’s releasing a tremendous amount of energy. They are so-named because of the prototype star used to research this phase of solar evolution – T Tauri, a variable star located in the direction of the Hyades cluster, about 600 light years from Earth.

A T Tauri star may be bright, but this all comes its gravitational energy from the collapsing material. The central temperature of a T Tauri star isn’t enough to support fusion at its core. Even so, T Tauri stars can appear as bright as main sequence stars. The T Tauri phase lasts for about 100 million years, after which the star will enter the longest phase of its development – the Main Sequence phase.

Main Sequence:

Eventually, the core temperature of a star will reach the point that fusion its core can begin. This is the process that all stars go through as they convert protons of hydrogen, through several stages, into atoms of helium. This reaction is exothermic; it gives off more heat than it requires, and so the core of a main sequence star releases a tremendous amount of energy.

This energy starts out as gamma rays in the core of the star, but as it takes a long slow journey out of the star, it drops down in wavelength. All of this light pushes outward on the star, and counteracts the gravitational force pulling it inward. A star at this stage of life is held in balance – as long as its supplies of hydrogen fuel lasts.

And how long does it last? It depends on the mass of the star. The least massive stars, like red dwarfs with half the mass of the Sun, can sip away at their fuel for hundreds of billions and even trillions of years. Larger stars, like our Sun will typically sit in the main sequence phase for 10-15 billion years. The largest stars have the shortest lives, and can last a few billion, and even just a few million years.

Red Giant:

Over the course of its life, a star is converting hydrogen into helium at its core. This helium builds up and the hydrogen fuel runs out. When a star exhausts its fuel of hydrogen at its core, its internal nuclear reactions stop. Without this light pressure, the star begins to contract inward through gravity.

This process heats up a shell of hydrogen around the core which then ignites in fusion and causes the star to brighten up again, by a factor of 1,000-10,000. This causes the outer layers of the star to expand outward, increasing the size of the star many times. Our own Sun is expected to bloat out to a sphere that reaches all the way out to the orbit of the Earth.

The temperature and pressure at the core of the star will eventually reach the point that helium can be fused into carbon. Once a star reaches this point, it contracts down and is no longer a red giant. Stars much more massive than our Sun can continue on in this process, moving up the table of elements creating heavier and heavier atoms.

White Dwarf:

A star with the mass of our Sun doesn’t have the gravitational pressure to fuse carbon, so once it runs out of helium at its core, it’s effectively dead. The star will eject its outer layers into space, and then contract down, eventually becoming a white dwarf. This stellar remnant might start out hot, but it has no fusion reactions taking place inside it any more. It will cool down over hundreds of billions of years, eventually becoming the background temperature of the Universe.

We have written many articles about the live cycle of stars on Universe Today. Here’s What is the Life Cycle Of The Sun?, What is a Red Giant?, Will Earth Survive When the Sun Becomes a Red Giant?, What Is The Future Of Our Sun?

Want more information on stars? Here’s Hubblesite’s News Releases about Stars, and more information from NASA’s imagine the Universe.

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?, Episode 13: Where Do Stars Go When they Die?, and Episode 108: The Life of the Sun.

Sources:

February 3, 2009May 4, 2017 by Fraser Cain

What is the Hottest Star?

Eta Carinae Credit: Gemini Observatory artwork by Lynette Cook

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Stars can range in temperature, from the relatively cool red dwarfs to superhot blue stars. So what is the hottest star in the Universe?

First, let’s talk a bit about temperature. The color of a star is a function of its temperature. If a star looks red, that means its surface temperature is approximately 2,500 Kelvin. Just for comparison, our Sun, which actually looks white from space, measures about 6,000 Kelvin. The hotter the star, the further up the spectrum you go. The hottest stars are the blue stars. A star appears blue once its surface temperature gets above 10,000 Kelvin, or so, a star will appear blue to our eyes.

So the hottest stars in the Universe are going to be a blue star, and we know they’re going to be massive. So the question is, how massive can stars get? One example is the star Rigel, in the constellation Orion. Rigel is thought to have 17 times the mass of the Sun, and puts out 40,000 times the luminosity of the Sun. It’s surface temperature is a mere 11,000 Kelvin. Another star in Orion, Bellatrix, has a temperature of 21,500 Kelvin. That’s even hotter.

But the hottest known stars in the Universe are the blue hypergiant stars. These are stars with more than 100 times the mass of the Sun. One of the best known examples is Eta Carinae, located about 7,500 light-years from the Sun. Eta Carinae could be as large as 180 times the radius of the Sun, and its surface temperature is 36,000-40,000 Kelvin.

Just for comparison, 40,000 Kelvin is about 72,000 degrees F.

So it’s the blue hypergiants, like Eta Carinae, which are probably the hottest stars in the Universe.

We have written many articles about stars on Universe Today. Here’s an article about how Eta Carinae is almost ready to explode as a supernova. And here’s a link to a nice photo of the nebula around Eta Carinae.

Want more information on stars? Here’s Hubblesite’s News Releases about Stars, and more information from NASA’s imagine the Universe.

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?

References:
NASA: Eta Carinae
University of Illinois