Universe Today

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?

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?

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

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

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When you look up into the night sky on a clear night in a place that has little light pollution, you can see thousands of stars spanning the sky, all of which lie in our galaxy, the Milky Way. The Milky Way seems really, really big when seen from the comparatively tiny Earth. What follows is a list of the attributes of our galactic neighborhood, the dimensions of our corner of space.

We’ll start with the mass of the Milky Way. It’s so massive that we have to give its mass in units of something rather large itself: the Sun. When you take into account all of the stars, gas, dust and the copious amounts of dark matter that surround our galaxy in a halo, it has about 3 trillion times the mass of the Sun, according to the most recent estimate as of this writing. Previous estimates put the number at over 1 trillion solar masses. Over 90% of that mass can be attributed to dark matter, matter that cannot be detected except for its gravitational pull.

Of course, the Milky Way isn’t all dark matter – there’s lots of gas, dust, and stars that populate the galactic disk. The number of stars in the Milky Way is estimated to be about 200-400 billion, though you can only see about 5,000-8,000 of those stars with the naked eye, and only about 2,500 of them at any one time from the Earth. For one of the most highly detailed images of our galaxy in all of its stars and splendor from the Spitzer Space Telescope, go here.

The Milky Way is a huge disk, roughly 100,000 to 120,000 light years across. Its thickness is 1,000 light years throughout most of the disk, but there is a spheroidal bulge at the center of the galaxy that is 12,000 light years in diameter. These proportions are similar to a small stack of DVDs with a rubber ball glued into the middle. For a great representation of the proportions of the Milky Way in these terms, check out the video Galaxies by the Bad Astronomer, Phil Plait.

If you want to get more details about the size of the Milky Way, check out the rest of our section on the Guide to Space and listen to Episode 99 of Astronomy Cast.

Source: NASA

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…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.

• What is the Brightest Star in the Universe?
• How Long Would it Take to Travel to the Nearest Star?
• How Does A Star Die?
• How Does a Star Form?
• What is a Binary Star?
• How Many Stars Can You See?
• How Many Stars?
• Brightest Stars
• Massive Stars
• Stars and Planets
• What is a Star?
• What is the Hottest Star?
• Life Cycle of Stars
• Core of a Star
• Color of Stars
• Star Main Sequence
• Wolf-Rayet Stars
• What Were the First Stars?
• Temperature of Stars
• Star Luminosity
• Star Distance
• What Are Stars Made Of?
• Birth of Stars
• Star Evolution
• Age of Stars
• Carbon Stars
• Star Charts
• Falling Stars
• Interesting Facts About Stars
• Glow in the Dark Stars
• Are there Green Stars?
• History of Stars
• How Long Do Stars Last?
• What is the Light From Stars?
• Nearest Stars
• Nuclear Fusion in Stars
• Young Stars
• Variable Stars
• Mass of Stars
• Size of Stars
• Why Do Stars Shine?
• Why Do Stars Die?
• Why Do Stars Twinkle?
• What is the Smallest Star?
• Pulsars
• Hertzsprung-Russell Diagram
• Chandrasekhar Limit
• How Far is the Nearest Star?
• Stellar Evolution
• Famous Stars
• Life of a Star
• Stellar Parallax
• Distance to Nearest Star
• Magnetar
• Accretion Disk
• Where are Stars Born?
• Why are Stars Different Colors?
• Star Cluster
• Examples of Stars
• Supernova
• The Different Types of Stars
• Do Stars Move
• Orion’s Belt Stars

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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.