STARS

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Are formed initially from gas and dust. They are composed mainly of the hydrogen gas.

Are very hot and give off huge amounts of energy in the form of heat and light.

Our Sun is a medium sized star.

Have a life-span of about 10 billion years, after which they will cease to exist.

Are very far away from Earth. The closest Star is about 23.5 trillion miles away.

Go through many stages in their lifetime. Some of the names for these stages are Red Giant, Planetary Nebulae, White Dwarf, Neutron and even Black Holes.

STARS

Each star in the sky is an enormous glowing ball of gas. Our sun is a medium-sized star.

Stars can live for billions of years. A star is born when an enormous cloud of hydrogen gas collapses until it is hot enough to burn nuclear fuel (producing tremendous amounts heat and radiation). As the nuclear fuel runs out (in about 5 billion years), the star expands and the core contracts, becoming a giant star which eventually explodes and turns into a dim, cool object (a black dwarf, neutron star, or black hole, depending on its initial mass). The largest stars have the shortest life span (still billions of years); more massive stars burn hotter and faster than their smaller counterparts (like the Sun).

The composition of stars is studied using spectroscopy in which their visible light (the spectrum) is studied.

GROUPS OF STARS In the universe, most stars occur in groups of at least two stars. Two stars that are locked in elliptical orbit around their center of mass (their barycenter) are called a binary star system. About half of all stars are in a binary star system.

GLOBULAR CLUSTERA globular star cluster is a spherical group of up to a million stars held together by gravity. These remote objects lie mostly around the central bulge of spiral galaxies.

There are larger groups of stars, called clusters. These are relatively unorganized collections of stars. An open cluster is a loose collection of up to about 1,000 stars. Examples of open clusters include the Pleiades and Hyades.

Huge, organized collections of stars are called galaxies. Our solar system is located in the Milky Way Galaxy, a spiral galaxy. For more in-depth information on galaxies, click here.

All groups of stars are held together by gravitational forces.

WHY ARE STARS HOT AND BRIGHT?Nuclear Fusion and NucleosynthesisStars are giant nuclear reactors. In the center of stars, atoms are taken apart by tremendous atomic collisions that alter the atomic structure and release an enormous amount of energy. This makes stars hot and bright. In most stars, the primary reaction converts hydrogen atoms into helium atoms, releasing an enormous amount of energy. This reaction is called nuclear fusion because it fused the nuclei (center) of atoms together, forming a new nucleus. The process of forming a new nucleus (and element) is nucleosynthesis.

For more information on nuclear fusion in stars, click here.

WHAT IS THE CLOSEST STAR?The closest star to us is the sun! Other than that, the closest star is Proxima Centauri, aka Alpha Centauri C (the dimmest star in the Alpha Centauri system). Proxima Centauri is 4.3 light-years from the Sun. It has an absolute magnitude of 15.5.

WHY DO STARS TWINKLE?The scientific name for the twinkling of stars is stellar scintillation (or astronomical scintillation). Stars twinkle when we see them from the Earth's surface because we are viewing them through thick layers of turbulent (moving) air in the Earth's atmosphere.

Stars (except for the Sun) appear as tiny dots in the sky; as their light travels through the many layers of the Earth's atmosphere, the light of the star is bent (refracted) many times and in random directions (light is bent when it hits a change in density - like a pocket of cold air or hot air). This random refraction results in the star winking out (it looks as though the star moves a bit, and our eye interprets this as twinkling).

Stars closer to the horizon appear to twinkle more than stars that are overhead - this is because the light of stars near the horizon has to travel through more air than stars overhead and subject to more refraction. Also, planets do not usually twinkle - they are big enough that this effect is not noticeable (except when the air is extremely turbulent).

Stars would not appear to twinkle if we viewed them from outer space (or from a planet/moon that didn't have an atmosphere).

STELLAR WINDStellar wind is ionized gas that is ejected from the surface of a star (including the Sun). Older (evolved) stars give off stronger stellar winds than younger stars.

StarsTop of Page

Looking up into the sky at night you can see many stars and other objects. Early mariners used the stars for navigation. Today we know much more about the stars, planets, Sun, moon and other objects in our sky. Stars change over time. It may take millions to billions of years for a star to live out its life. That is a very, very long time! Stars are formed initially from gas and dust. They are composed mainly of the hydrogen gas. They are very hot and give off huge amounts of energy in the form of heat and light.

Our Sun is a medium sized star and has a life span of about 10 billion years, after which it will cease to exist. Stars are very far away from the Earth. The closest Star is about 23.5 trillion miles away.

Stars in the SkyTop of Page

Stars are contained in galaxies. A galaxy contains not only stars, but clouds of gas and dust. There are billions of stars in the sky, which are far away from the Earth. You can see the stars as twinkling dots of light in the sky. Almost 2000 stars can be seen with your eyes and more stars can be seen with binoculars and telescopes. A star is a big ball of gas, which gives off both heat and light.

Twinkling of StarsTop of Page

Why do stars twinkle? Light comes from the stars in straight lines. When light comes to the Earth, it passes through layers of air that form the Earths' atmosphere. The air makes the light bent many times and in random directions. We will see the stars twinkling, when the light bends from the stars as it goes through the air.

Stars at Day and NightTop of Page

Why can we see the sun but seldom see the stars during the daytime? Stars are always in the sky. Our sun makes so much light, it hides the other stars in the day time. At night, when the sun is not there in our part of the sky, we can see the other stars.

Star PatternsTop of Page

The Stars you see at night are very much like the Sun, but they are very far away. Hence, they appear to be smaller than the Sun. Stars appear in groups and single. A group of stars forms a constellation. You see different constellations at different times of the year. People in different parts of the world assumed different shapes and gave them different names.

One of the numerous constellations that we see in the night sky is the Big Dipper or the Great Bear; it is made up of 7 stars. Four stars form a quadrilateral, to form the body of the bear and 3 stars form the tail. These star patterns appear to move across the sky as the Earth rotates. As the Earth revolves around the Sun, you see these star patterns in different parts of the sky.

Stars and SeasonsTop of Page

In spring the big dipperis high in the sky and looks upside down.

In summer the big dippermoves down with its handle pointing up.

In autumn the big dipperis low and looks as if it can hold water.In winter the big dipperhas moved up with its handle pointing down.

Stars

Lights in the Sky

HYPERLINK "http://www.seasky.org/celestial-objects/" \l "Section 2" A Nuclear Furnace

HYPERLINK "http://www.seasky.org/celestial-objects/" \l "Section 3" The Circle of Life

HYPERLINK "http://www.seasky.org/celestial-objects/" \l "Section 4" Types of Stars

Lights in the SkyFor eons, mankind has looked to the heavens and wondered at the lights in the sky. Ancient people believed they could see shapes among the stars. They identified both animals and people, and each had its own story. These chance alignments of the stars are known as constellations. Today, the 88 constellations are used by astronomers to organize the night sky and to identify the locations of the stars. Stars are the most plentiful objects in the visible universe. They provide the light and energy that fuels a solar system. They also create the heavy elements that are necessary to form life. Without stars, there would be no life. The Sun provides energy for nearly every living thing on Earth. It also warms our planet's surface to create a virtual oasis in the coldness of space. A star's brightness is known as its magnitude. Astronomers rate the magnitude of a star with a scale that gives brighter stars a low number and dim stars a higher number. Each whole number on this scale is 10 times dimmer than the previous number. The brightest star the night sky is Sirius. With a magnitude of -1.46, it is almost 15 times brighter than a star with a magnitude of zero. Stars with a magnitude of 8 or more are too dim to see with the naked eye. Stars are identified by their color, which indicates their temperature. They are divided into what are known as spectral classes. These classes are O, B, A, F, G, K, and M. Class O stars are the hottest and are blue in color. The coolest stars are identified as class M and are red in color.

Contrary to popular belief, stars do not actually twinkle. This phenomenon is a result of atmospheric interference. The effect is similar to what takes place on a hot summer day when you look across hot pavement or a parking lot. The rising air causes images to waver. This is what causes the twinkling effect in stars. The lower a star is in the sky, the more it will twinkle because its light must pass through more of the atmosphere.A Nuclear FurnaceA star is like a gigantic nuclear furnace. The nuclear reactions inside convert hydrogen into helium by means of a process known as fusion. It is this nuclear reaction that gives a star its energy.

A few things to be aware of to understand the tables in the book:

1. Brightness is measured in Magnitudes (the smaller the magnitude the brighter the star). This is how bright it looks to your eye. On a dark night in a remote area we can see with the naked eye stars of 6th magnitude or brighter. In a city, we can see stars of 1st, 2nd or 3rd magnitude or brighter. A star can be bright either because it is near by, or because it is really bright. All of the stars you mention are much brighter than the Sun (Sirius is 24 times brighter -- table A4). If someone lived on a planet near those stars, they would not be able to see the Sun with their naked eye.

2. Temperature is measured in degrees Kelvin (= degrees Celsius + 273), or by color or by "spectral type". The spectra types are, from hottest to coolest, O B A F G K M. These have historically been remembered by, "Oh Be A Fine Girl/Guy Kiss Me." Later three types of strange cool stars were added as R, N and S for Right Now Smack. See table A-3 for a conversion between each of these.

3. Mass is measured in "solar masses", so the Sun has a mass of 1 solar mass.

4. Distance is often measured in light years (how long it takes light to travel in one year -- or about a Million years by jet airliner).

Now, back to your question of age. The more massive a star is the brighter it is, thus the faster it runs out of fuel. So, in general, the more massive a star is the shorter its life-span. So a star like Rigel must be quite young, as compared to our Sun (which is 4 billion years old).

In general, the exact age of one star is quite hard to measure. If there is a cluster of stars, you can assume they are all the same age, so we can find the age of all the stars. Even this is sometime hard to do too, because the exact age depends on many things, including the exact chemicals that make up the stars.

The first thing you need to know is that constellations are not real!

The constellations are totally imaginary things that poets, farmers and astronomers have made up over the past 6,000 years (and probably even more!). The real purpose for the constellations is to help us tell which stars are which, nothing more. On a really dark night, you can see about 1000 to 1500 stars. Trying to tell which is which is hard. The constellations help by breaking up the sky into more managable bits. They are used as mnemonics, or memory aids. For example, if you spot three bright stars in a row in the winter evening, you might realize, "Oh! That's part of Orion!" Suddenly, the rest of the constellation falls into place and you can declare: "There's Betelgeuse in Orion's left shoulder and Rigel is his foot." And once you recognize Orion, you can remember that Orion's Hunting Dogs are always nearby. Then you might recognize the two bright stars in the upper and lower left of the photograph as Procyon in Canis Minor and Sirius in Canis Major, respectively.

Star

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This article is about the astronomical object. For other uses, see Star (disambiguation).

The Pleiades, an open cluster of stars in the constellation of Taurus. NASA photoA star is a massive, luminous ball of plasma that is held together by gravity. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth. Other stars are visible in the night sky, when they are not outshone by the Sun. For most of its life, a star shines due to thermonuclear fusion in its core releasing energy that traverses the star's interior and then radiates into outer space. Almost all elements heavier than hydrogen and helium were created by fusion processes in stars. Astronomers can determine the mass, age, chemical composition and many other properties of a star by observing its spectrum, luminosity and motion through space. The total mass of a star is the principal determinant in its evolution and eventual fate. Other characteristics of a star are determined by its evolutionary history, including the diameter, rotation, movement and temperature. A plot of the temperature of many stars against their luminosities, known as a Hertzsprung-Russell diagram (HR diagram), allows the age and evolutionary state of a star to be determined.

A star begins as a collapsing cloud of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements. Once the stellar core is sufficiently dense, some of the hydrogen is steadily converted into helium through the process of nuclear fusion.[1] The remainder of the star's interior carries energy away from the core through a combination of radiative and convective processes. The star's internal pressure prevents it from collapsing further under its own gravity. Once the hydrogen fuel at the core is exhausted, those stars having at least 0.4 times the mass of the Sun[2] expand to become a red giant, in some cases fusing heavier elements at the core or in shells around the core. The star then evolves into a degenerate form, recycling a portion of the matter into the interstellar environment, where it will form a new generation of stars with a higher proportion of heavy elements.[3]Binary and multi-star systems consist of two or more stars that are gravitationally bound, and generally move around each other in stable orbits. When two such stars have a relatively close orbit, their gravitational interaction can have a significant impact on their evolution.[4] Stars can form part of a much larger gravitationally bound structure, such as a cluster or a galaxy.