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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.
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.
The life cycle of a Sun-like star, from its birth on the left side of the frame to its evolution into a red giant on the right after billions of years. Credit: ESO/M. Kornmesser
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.
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.
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The color of a star is defined by its temperature. The coolest stars are red, while the hottest stars are blue. And the temperature of a star is defined by its mass. The most massive stars in the Universe are the blue supergiant stars; then can have more than 20 times the mass of the Sun. Blue giant stars are very hot, with surface temperatures of 20,000-50,000 Kelvin. Just for comparison, our own Sun is only 6,000 Kelvin.
Blue supergiant star have extremely high masses, sometimes with dozens of times the mass of the Sun. They form in the largest, most active star forming regions where large amounts of mass can come together to form the biggest stars: star clusters, the arms of spiral galaxies and in irregular galaxies.
Perhaps the best known example of a blue supergiant star is Rigel, located in the constellation Orion. It has about 20 times the mass of the Sun, and puts out 60,000 times as much energy.
Blue supergiants can turn into red supergiants and vice versa. When the star is smaller and more compact, its luminosity is contained over a smaller surface area and so its temperature is much hotter; this is the blue supergiant phase. These stars can then puff up expanding to a much larger size, spreading their luminosity over a much larger area. Then they become red supergiant stars, and appear the cooler red color. Astronomers think supergiants can fluctuate back and forth between red and blue supergiant, puffing off an outer layer of material with each contraction.
Eventually a supergiant runs out of material to continue supporting fusion in its core, and will detonate as a supernova – one of the brightest explosions in the Universe.
We have written many articles about stars on Universe Today. Here’s an article that talks about the constellation Orion, including the star Rigel, and here’s a nice picture of Rigel passing behind Saturn.
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Stars come in many shapes and sizes and they come in many colors. Some of the hottest stars in the Universe are blue giant stars. You see, the color of a star is defined by its temperature; the coolest stars are red, while the hottest ones appear blue. And the temperature of a star comes from its mass. The more massive a star, the hotter it’s going to be. Stars don’t get more more massive or hot than blue giant stars.
Blue giants blaze with a surface temperature of 20,000 Kelvin or more, and are extremely luminous. Just for comparison, a star like our Sun only has a surface temperature of about 6,000 Kelvin. A blue giant star can put out 10,000 times as much energy as the Sun. Astronomers categorize blue giants as type O or B stars, belonging to the luminosity class III. The can reach an absolute magnitude of -5 or -6.
The true monsters of the Universe are blue supergiant stars, like Rigel. These can be a blue star with surface temperatures of 20,000 – 50,000 Kelvin and can be 25 times larger than the Sun. Because they’re so large, and burn so hot, they use up their fuel very quickly. A middle-sized star like our Sun might last for 12 billion years, while a blue supergiant will detonate with a few hundred million years. The smaller stars will leave neutron stars or black holes behind, while the largest will just vaporize themselves completely.
We have written many articles about stars on Universe Today. Here’s an article that looks for the biggest star in the Universe.
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Spend any time watching the night sky and you’ll soon recognize that stars have different colors. Some are white, others yellow or red, and some are blue. Blue stars are made of the same stuff as all the other stars in the Universe; they’re about 75% hydrogen and 24% helium with trace amounts of other elements. So what makes a blue star… blue?
The color of a star comes from its temperature. The coolest stars appear red, while the hottest stars are blue. And for a star, the only thing that defines the temperature of a star is its mass. Blue stars are stars that have at least 3 times the mass of the Sun and up. Whether a star has 10 times the mass of the Sun or 150 solar masses, it’s going to appear blue to our eyes.
An example of a blue star is the familiar Rigel, the brightest star in the constellation Orion and the 6th brightest star in the sky. Astronomers calculate that Rigel is approximately 700 and 900 light-years away, and yet it appears almost as bright as a star like Sirius which is only 8.3 light-years away. The temperature of Rigel is approximately 11,000 Kelvin; it’s this high temperature that accounts for Rigel’s color. Rigel puts out about 40,000 times the energy of the Sun.
An even more extreme example of a blue star is the blue supergiant Eta Carinae, located about 8,000 light-years away in the Carina constellation. Again, Eta Carinae is 10 times further away than Rigel, and yet from our perspective it’s only a little dimmer. The surface temperature of Eta Carinae is 40,000 Kelvin, and it shines with much of its radiation in the ultraviolet spectrum. Since this wavelength is invisible to our eyes, we perceive it as blue. All told, Eta Carinae is blasting out 1,000,000 times the energy of our Sun.
Blue stars burn through their fuel at a tremendous rate. With 150 times the mass of the Sun, Eta Carinae has only been around for a few million years and it’s expected to detonate as a supernova within the next 100,000 years. Our Sun, in comparison, has been around for 4.5 billion years and is expected to live another 7 billion years.
So, remember, blue stars are blue because of the temperature of their surface. And they’re so hot because blue stars are much more massive than cooler stars like our Sun.
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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.
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Our Sun is a star. It’s a vast ball made up of 74% hydrogen and 24% helium, with trace amounts of other elements. It has so much mass that the temperatures and pressures at its core are hot enough to ignite fusion. At the core of the Sun (and other stars), atoms of hydrogen are being fused into atoms of helium. This process releases a tremendous amount of energy. If an object isn’t performing some kind of fusion at its core, it’s not a star.
Most planets are actually made of similar material to the Sun. Both Jupiter and Saturn have similar mixtures of hydrogen and helium. If the planet Jupiter is made of hydrogen, why doesn’t it shine like a star? It all comes down to mass. Jupiter would need to be about 80 times more massive before it had enough mass to actually ignite hydrogen fusion at its core.
The small rocky terrestrial planets like the Earth and Mars make up just a fraction of the mass of the Solar System. Unlike the larger gas giants, the terrestrial planets are mostly made up of denser elements, like iron, silicon and oxygen. The larger gas giant planets probably have large quantities of these heavier elements in their cores. In fact, Jupiter probably has an Earth-like ball of rock with 14 to 18 times the mass of the Earth at its core.
What about orbits? Planets orbit stars, no question. But you can also have multi-star systems where stars are orbiting stars. And it’s also possible that you could have binary planets orbiting a common center of gravity and together they orbit around a star.
The end of the day, the only real difference between planets and stars is mass – almost everything out there is made up of 75% hydrogen and 24% helium. If an object has about 80 times the mass of Jupiter, it has sufficient mass and temperature to ignite solar fusion in its core. If it doesn’t… it can’t
Eta Carinae Credit: Gemini Observatory artwork by Lynette Cook
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Since they’re all just made of hydrogen and helium, when it comes to stars, mass is everything. The amount of mass that a star has defines its luminosity, size and even how long it will live. The most massive stars in the Universe really live fast and die hard; they can amass more than 100 times the mass of the Sun, and will only live a few million years before detonating as supernovae.
How massive is massive? Some astronomers think that the theoretical limit for stellar mass is about 150 times the mass of the Sun (1 solar mass is the mass of the Sun), beyond this limit powerful stellar winds will push away infalling material before it can join the star. And stars with 150 solar masses have been observed, at least theoretically.
The most accurate way to measure the mass of an object like a star is if it’s in a binary system with another object. Astronomers can calculate the mass of the two objects by measuring how they orbit one another. But the most massive stars ever seen don’t have any binary companions, so astronomers have to guess at how massive they are. They estimate the star’s mass based on its temperature and absolute brightness.
There are dozens of known stars estimated to have 25 times the mass of the Sun. Here’s a list of the most massive known stars:
HD 269810 (150 solar masses)
Peony Nebula Star (150 solar masses)
Eta Carinae (150 solar masses)
Pistol Star (150 solar masses)
LBV 1806-20 (130 masses)
All of these stars are supergiant stars, which formed inside the largest clouds of gas and dust. Stars this large aren’t long for the Universe. They burn tremendous amounts of fuel and can be 500,000 times more luminous than the Sun.
Perhaps the most familiar, extremely massive star is Eta Carinae, located about 8,000 light years from Earth. Astronomers think it has an estimated mass of between 100 and 150 solar masses. The star is probably less than 3 million years old, and it’s believed that it has less than 100,000 years left to live. When it detonates, Eta Carinae’s supernova will be bright enough to see in the day, and you could read a book with it at night.
