What Are The Different Types of Stars?

Artist's depiction of the Morgan-Keenan spectral diagram, showing how stars differ in colors as well as size. Credit: Wikipedia Commons

A star is a star, right? Sure there are some difference in terms of color when you look up at the night sky. But they are all basically the same, big balls of gas burning up to billions of light years away, right?  Well, not exactly. In truth, stars are about as diverse as anything else in our Universe, falling into one of many different classifications based on its defining characteristics.

All in all, there are many different types of stars, ranging from tiny brown dwarfs to red and blue supergiants. There are even more bizarre kinds of stars, like neutron stars and Wolf-Rayet stars. And as our exploration of the Universe continues, we continue to learn things about stars that force us to expand on the way we think of them. Let’s take a look at all the different types of stars there are.

Protostar:

A protostar is what you have before a star forms. A protostar is a collection of gas that has collapsed down from a giant molecular cloud. The protostar phase of stellar evolution lasts about 100,000 years. Over time, gravity and pressure increase, forcing the protostar to collapse down. All of the energy release by the protostar comes only from the heating caused by the gravitational energy – nuclear fusion reactions haven’t started yet.

Size chart showing our Sun (far left) compared to larger stars. Credit: earthspacecircle.blogspot.ca
Size chart showing our Sun (far left) compared to larger stars. Credit: earthspacecircle.blogspot.ca

T Tauri Star:

A T Tauri star is stage in a star’s formation and evolution right before it becomes a main sequence star. This phase occurs at the end of the protostar phase, when the gravitational pressure holding the star together is the source of all its energy. T Tauri stars don’t have enough pressure and temperature at their cores to generate nuclear fusion, but they do resemble main sequence stars; they’re about the same temperature but brighter because they’re a larger. T Tauri stars can have large areas of sunspot coverage, and have intense X-ray flares and extremely powerful stellar winds. Stars will remain in the T Tauri stage for about 100 million years.

Main Sequence Star:

The majority of all stars in our galaxy, and even the Universe, are main sequence stars. Our Sun is a main sequence star, and so are our nearest neighbors, Sirius and Alpha Centauri A. Main sequence stars can vary in size, mass and brightness, but they’re all doing the same thing: converting hydrogen into helium in their cores, releasing a tremendous amount of energy.

A star in the main sequence is in a state of hydrostatic equilibrium. Gravity is pulling the star inward, and the light pressure from all the fusion reactions in the star are pushing outward. The inward and outward forces balance one another out, and the star maintains a spherical shape. Stars in the main sequence will have a size that depends on their mass, which defines the amount of gravity pulling them inward.

The lower mass limit for a main sequence star is about 0.08 times the mass of the Sun, or 80 times the mass of Jupiter. This is the minimum amount of gravitational pressure you need to ignite fusion in the core. Stars can theoretically grow to more than 100 times the mass of the Sun.

Red Giant Star:

When a star has consumed its stock of hydrogen in its core, fusion stops and the star no longer generates an outward pressure to counteract the inward pressure pulling it together. A shell of hydrogen around the core ignites continuing the life of the star, but causes it to increase in size dramatically. The aging star has become a red giant star, and can be 100 times larger than it was in its main sequence phase. When this hydrogen fuel is used up, further shells of helium and even heavier elements can be consumed in fusion reactions. The red giant phase of a star’s life will only last a few hundred million years before it runs out of fuel completely and becomes a white dwarf.

White Dwarf Star:

When a star has completely run out of hydrogen fuel in its core and it lacks the mass to force higher elements into fusion reaction, it becomes a white dwarf star. The outward light pressure from the fusion reaction stops and the star collapses inward under its own gravity. A white dwarf shines because it was a hot star once, but there’s no fusion reactions happening any more. A white dwarf will just cool down until it becomes the background temperature of the Universe. This process will take hundreds of billions of years, so no white dwarfs have actually cooled down that far yet.

Red Dwarf Star:

Red dwarf stars are the most common kind of stars in the Universe. These are main sequence stars but they have such low mass that they’re much cooler than stars like our Sun. They have another advantage. Red dwarf stars are able to keep the hydrogen fuel mixing into their core, and so they can conserve their fuel for much longer than other stars. Astronomers estimate that some red dwarf stars will burn for up to 10 trillion years. The smallest red dwarfs are 0.075 times the mass of the Sun, and they can have a mass of up to half of the Sun.

Neutron Stars:

If a star has between 1.35 and 2.1 times the mass of the Sun, it doesn’t form a white dwarf when it dies. Instead, the star dies in a catastrophic supernova explosion, and the remaining core becomes a neutron star. As its name implies, a neutron star is an exotic type of star that is composed entirely of neutrons. This is because the intense gravity of the neutron star crushes protons and electrons together to form neutrons. If stars are even more massive, they will become black holes instead of neutron stars after the supernova goes off.

Supergiant Stars:

The largest stars in the Universe are supergiant stars. These are monsters with dozens of times the mass of the Sun. Unlike a relatively stable star like the Sun, supergiants are consuming hydrogen fuel at an enormous rate and will consume all the fuel in their cores within just a few million years. Supergiant stars live fast and die young, detonating as supernovae; completely disintegrating themselves in the process.

As you can see, stars come in many sizes, colors and varieties. Knowing what accounts for this, and what their various life stages look like, are all important when it comes to understanding our Universe. It also helps when it comes to our ongoing efforts to explore our local stellar neighborhood, not to mention in the hunt for extra-terrestrial life!

We have written many articles about stars on Universe Today. Here’s What is the Biggest Star in the Universe?, What is a Binary Star?, Do Stars Move?, What are the Most Famous Stars?, What is the Brightest Star in the Sky, Past and Future?

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?

Neutron Star

Neutron stars are formed when large stars run out of fuel and collapse. To get a neutron star, you need to have star that’s larger than about 1.5 solar masses and less than 5 times the mass of the Sun.

If you have less than 1.5 solar masses, you don’t have enough material and gravity to compress the object down enough. You only get a white dwarf. This is what will happen to our own Sun one day.

If you have more than 5 times the mass of the Sun, your star will end up as a black hole.

But if your star is right in between those masses, you get a neutron star.

The neutron star is formed when the star runs out of fuel and collapses inward on itself. The protons and electrons of atoms are forced together into neutrons. Since the star still has a lot of gravity, any additional material falling into the neutron star is super-accelerated by the gravity and turned into identical neutron material.

Just one teaspoon of a neutron star would have the mass of over 5 x 1012 kilograms.

A neutron star actually has different layers. Astronomers think there’s an outer shell of atomic nuclei with electrons about 1 meter thick. Below this crust, you get nuclei with increasing numbers of neutrons. These would decay quickly on Earth, but the intense pressure of the gravity keeps them stable.

When neutron stars form, they maintain the momentum of the entire star, but now they’re just a few kilometers across. This causes them to spin at tremendous rates, sometimes as fast as hundreds of times a second.

We have written many articles about stars on Universe Today. Here’s an article about a neutron star with a tail like a comet, and here’s an article about a a shooting 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?

What is a Shooting Star?

A shooting star is another name for a meteoroid that burns up as it passes through the Earth’s atmosphere. So, a shooting star isn’t a star at all.

Most of the shooting stars that we can see are known as meteoroids. These are objects as small as a piece of sand, and as large as a boulder. Smaller than a piece of sand, and astronomers call them interplanetary dust. If they’re larger than a boulder, astronomers call them asteroids.

A meteoroid becomes a meteor when it strikes the atmosphere and leaves a bright tail behind it. The bright line that we see in the sky is caused by the ram pressure of the meteoroid. It’s not actually caused by friction, as most people think.

When a meteoroid is larger, the streak in the sky is called a fireball or bolide. These can be bright, and leave a streak in the sky that can last for more than a minute. Some are so large they even make crackling noises as they pass through the atmosphere.

If any portion of the meteoroid actually survives its passage through the atmosphere, astronomers call them meteorites.

Some of the brightest and most popular meteor showers are the Leonids, the Geminids, and the Perseids. With some of these showers, you can see more than one meteor (or shooting star) each minute.

We have written many articles about stars on Universe Today. Here’s an article about the Quadrantid meteor shower, and here’s an article about the Geminids.

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?

What is a Binary Star?

Young binarys stars: Image credit: NASA

The term binary star is a misnomer because it is actually a star system made up of usually two stars that orbit around one center of mass – where the mass is most concentrated. A binary star is not to be confused with two stars that appear close together to the naked eye from Earth, but in reality are very far apart – Carl Sagan far!

Astrophysicists find binary systems to be quite useful in determining the mass of the individual stars involved. When two objects orbit one another, their mass can be calculated very precisely by using Newton’s calculations for gravity. The data collected from binary stars allows astrophysicists to extrapolate the relative mass of similar single stars.

There are several subcategories of binary stars, classified by their visual properties including eclipsing binaries, visual binaries, spectroscopic binaries and astrometric binaries.

Eclipsing binary stars are those whose orbits form a horizontal line from the point of observation; essentially, what the viewer sees is a double eclipse along a single plane; Algol for example.

A visual binary system is a system in which two separate stars are visible through a telescope that has an appropriate resolving power. These can be difficult to detect if one of the stars’ brightness is much greater, in effect blotting out the second star.

Spectroscopic binary stars are those systems in which the stars are very close and orbiting very quickly. These systems are determined by the presence of spectral lines – lines of color that are anomalies in an otherwise continuous spectrum and are one of the only ways of determining whether a second star is present. It is possible for a binary star system to be both a visual and a spectroscopic binary if the stars are far enough apart and the telescope being used is of a high enough resolution.

Astrometric binary stars are systems in which only one star can be observed, and the other’s presence is inferred by the noticeable wobble of the first star. This wobble happens as a result of the smaller star’s slight gravitational influence on the larger star.

So now you can answer the question, “what is a binary star?”

We have written many articles about binary stars on Universe Today. Here’s an article about a new class of binary stars discovered, and a situation where one star was ejected out of a binary partnership.

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?

Source: NASA

What is the North Star?

Were you wondering about the North Star? Firstly, you might expect one of the most famous stars in the night sky to be one of the brightest, but it isn’t; not by a long shot. That honor belongs to Sirius and many less bright stars besides. The North Star shines with a humble brightness that belies its navigational importance.

Polaris, or the North Star, sits almost directly above the North Pole; therefore, it is a reliable gauge of North if you find yourself lost on a clear night without a compass. Stars that sit directly above the Earth’s North or South Pole are called Pole Stars. Interestingly, the North Star hasn’t always been, nor will it always be the Pole Star because the Earth’s axis changes slightly over time, and stars move in relation to each other over time.

You can also approximate your latitude by measuring the angle of elevation between the horizon and the North Star. There is no equivalent star in the South Pole, but Sigma Octantis comes close. It isn’t very useful for navigational purposes as it isn’t very bright to the naked eye. Instead, navigators use two of the stars in the Southern Cross, Alpha and Gamma to determine due South.

The North Star is easy to find if you can first locate the Little Dipper. The North Star lies at the end of the handle in the Little Dipper (Ursa Minor). For a point of reference, The Big Dipper (Ursa Major) lies below the little dipper and their handles point in opposite directions. The two stars in the end of the ladle of The Big Dipper point to Polaris. Also, both The Big Dipper and The Little Dipper remain in the sky all night long, rotating in relation to the Earth’s axis.

We have written many articles about stars on Universe Today. Here’s an article that talks more about how the North Star is actually a variable star. And it’s really three stars in one.

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://stars.astro.illinois.edu/sow/polaris.html
http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/980203a.html

How Does a Star Die?



So a star has reached middle age by fusing hydrogen into helium. Then what happens? Once a star has run out of usable hydrogen that it can convert into helium, a star then takes one of several paths.

If the star is 0.5 solar masses (half the mass of our sun), electron degeneracy pressure will prevent the star from collapsing in upon itself. Due to the age of the universe, scientists can only use computer modeling to predict what will happen to such a star. Once it has finished its active phase (hydrogen to helium), it becomes a white dwarf.

A white dwarf can come about in one of two ways; first, if the star is very small, electron degeneracy pressure simply stops the collapse of the star, it is out of hydrogen, and it becomes a white dwarf. Secondly, and more commonly, the core of the star can still be surrounded by some layers of hydrogen, which continue to fuse and cause the star to expand, becoming a red giant.

A red giant is a star in the process of fusing helium to form carbon and oxygen. If there is insufficient energy to make this happen, the outer shell of the star will shed leaving behind an inert core or oxygen and carbon – a remnant white dwarf. If enough energy is involved in the casting off of stellar casings, a nebula can form. If said white dwarf is in a binary system, it could become a type 1A supernova, but this is very rare. Instead, it is thought that a white dwarf will eventually cool to become a black dwarf – in theory because there are no white dwarfs older than the universe, black dwarfs are theoretical only because there hasn’t been enough time for one to form.

If a star that has reached the end of its productive phase is below the Chandrasekhar Limit – 1.4 times the mass of our Sun – it will become a white dwarf; over this limit, it will become a neutron star. If a star is larger than about 5 times the mass of the sun, when the hydrogen fusing stops, a supernova will take place and the rest of the material will condense into a black hole.

We have written many articles about stars on Universe Today. Here’s an article with photographs of a star’s death captured by the Chandra X-Ray Observatory, and here’s an article about a hypergiant star nearing death.

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?

Source: NASA

Stars

Stars

Stars…you see thousands of them every time you look into the night sky. Well, that is if you bother to even notice the. They are sort of like trees, or houses; they are there every time you look, so most people take their presence for granted and never give them a second thought. To help you understand what you are looking at in the sky, here are a few fun facts about stars followed by a long list of links to articles about them.

When you look into the night sky, all of the stars appear to be white, but they are not. Stars come in many colors: blue, brown, yellow, red, and orange to name a few. Within each of those colors there are several subcategories like giant and dwarf and a few ways to classify the age of a star.

Stars create energy in one of two ways. The first is converting hydrogen to helium in a proton-proton chain reaction basis(P-P) or the CNO cycle where they convert carbon to nitrogen to oxygen to convert hydrogen to helium(CNO cycle).

Our Sun is a single star. It stands alone near the barycenter of our Solar System. That gives some people the impression that this is how things are every where in the universe, but many stars occur in groups. There are many binary(two) star systems and some known to have as many as 6 in a system.

In the links below you will find thousands of facts about stars. Mixed in with the facts are images and a few other things. Enjoy your reading.

Orbit of Saturn

Saturn, seen by Cassini. Image credit: NASA/JPL/SSI

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The orbit of Saturn lasts 29.7 years. In other words, during the time Saturn completes one full revolution around the Sun, Earth has gone through almost 30 years.

Like all the planets in the Solar System, the orbit of Saturn isn’t a perfect circle. It follows an elliptical path around the Sun.

The closest point of Saturn’s orbit is called its perihelion. At this point, Saturn is only 1.353 billion km or 9 astronomical units from the Sun (1 AU is the average distance from the Earth to the Sun).

The most distant point of Saturn’s orbit is called aphelion. At this point, Saturn is 1.513 billion km or 10.1 astronomical units from the Sun.

One of the interesting features about Saturn’s orbit is our perspective from here on Earth. Like Earth, Saturn’s axis of rotation is inclined compared to the plane of the Sun. For half of its orbit, Saturn’s southern pole faces the Sun, and then its northern pole faces the Sun for the other half of its orbit. And over the course of the year, there are times when we have a full view of Saturn’s rings, and other times when the rings are seen edge on.

Since Saturn takes almost 30 years to complete an orbit around the Sun, it has only gone around the Sun about 13 times since Galileo first observed Saturn in a telescope in 1610.

We have written many articles about Saturn for Universe Today. Here’s an article about how Saturn’s rings “disappear” as it orbits the Sun.

Want more information on Saturn? Here’s a link to Hubblesite’s News Releases about Saturn, and here’s NASA’s Solar System Exploration Guide.

We have recorded a podcast just about Saturn for Astronomy Cast. Click here and listen to Episode 59: Saturn.

Rotation of Saturn

Saturn Compared to Earth. Image credit: NASA/JPL

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Measuring the rotation of Saturn is actually a more complicated job than you might think. That’s because Saturn is just a ball of hydrogen and helium, without any solid surface features that you can measure from day to day. Saturn’s rotation is even more complicated than that, since different parts of the planet rotate at different speeds. So asking what the rotation of Saturn is depends on which part of the planet you’re talking about.

The visible features of Saturn rotate at different rates depending on their latitude (distance from the equator). Astronomers have developed three different systems for measuring the rotational speed of Saturn. System I is for regions around the planet’s equator. The System I rotation speed is 10 hours and 14 minutes. Above and below the Equatorial Belt is called System II. Here the rotation speed is 10 hours and 39 minutes.

System III is based on the rotation of Saturn’s magnetic field, and was measured by NASA’s voyager spacecraft. They determined that Saturn’s magnetic field takes 10 hours and 39 minutes to complete a rotation. But here’s a strange mystery. The rotation of the magnetic field was measured again by NASA’s Cassini spacecraft in 2004, and it found that the rotation of the magnetic field had slowed down to 10 hours and 45 minutes. So it appears that the rotation of Saturn can change over time.

We have written many articles about Saturn for Universe Today. Here’s an article that goes into much more detail about the process of measuring a day on Saturn.

Want more information on Saturn? Here’s a link to Hubblesite’s News Releases about Saturn, and here’s NASA’s Solar System Exploration Guide.

We have recorded a podcast just about Saturn for Astronomy Cast. Click here and listen to Episode 59: Saturn.

Saturn Compared to Earth

Saturn Compared to Earth. Image credit: NASA/JPL

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Saturn is the second largest planet in the Solar System (after Jupiter), but you really need a comparison. Let’s take a look at Saturn compared to Earth.

First, let’s just look at Saturn’s physical characteristics. The equatorial diameter of Saturn is 120,536 km; that’s about 9.5 times bigger than the diameter of the Earth. The surface area of Saturn is 83 times the area of Earth, and the volume is 764 times the volume of Earth. In other words, you could fit 764 planets the size of Earth inside Saturn. Finally, the mass of Saturn is 95 times the mass of the Earth.

One interesting comparison between Earth and Saturn is density. Earth is the densest planet in the Solar System, while Saturn is the least dense. The density of Earth is 5.52 g/cm3, while the density of Saturn is 0.687 g/cm3. In other words, Earth is 8 times as dense as Saturn.

Another region where Saturn and Earth are similar is gravity. Of course, Saturn has much more mass than Earth, but it’s spread out over a larger area. Saturn doesn’t have a solid surface, of course, but if you could walk on the surface of Saturn, you would experience almost exactly the same gravity as you feel on Earth.

Earth takes 24 hours to complete a day, while Saturn takes 10 hours and 32 minutes. A year on Earth is, well, 1 year, while a year on Saturn lasts 30 years.

Are you wondering about other planets compared to Earth? Here’s an article about Jupiter compared to Earth, and here’s Mars compared to Earth.

Want more information on Saturn? Here’s a link to Hubblesite’s News Releases about Saturn, and here’s NASA’s Solar System Exploration Guide.

We have recorded a podcast just about Saturn for Astronomy Cast. Click here and listen to Episode 59: Saturn.