STARSby Baylor Didier

What Is A Star ?A star is a sphere of gas held together by its own gravity.

There is a constant battle between the force of gravity, which is trying to cause the star to collapse, and the pressure of hot gas and radiation in the star, which is trying to expand outwards.

Life Cycle of a Star

Nebulas are giant clouds of gas and dust where stars are born.

The amount of gas and dust in the nebula determines the mass of the star.

The mass of the star determines the star’s life cycle.

Life Cycle of a StarAfter millions of years, the hydrogen gas in the nebula is pulled together by gravity and it begins to spin faster and faster, heating up and becoming a young star called a protostar. Eventually the temperature reaches 15 million degrees and nuclear fusion begins within the star. The star begins to glow, contracts, and becomes stable. It is now a main sequence star which will shine for millions to billions of years. This is the stage our Sun is at right now.

Life Cycle of a StarDuring most of it’s lifetime, the star’s interior heat and radiation is supplied by nuclear reactions near the center. This phase of the star's lifecycle is called the main sequence.

During the main sequence, hydrogen is converted into helium by nuclear fusion.

Life Cycle of a StarWhen the hydrogen supply in the star runs out, and the star is no longer generating enough heat by nuclear fusion, the core becomes unstable and contracts.

The outer shell of the star starts to expand, cools, and glows red. The star has now reached the red giant phase.

In the core of the red giant, helium fuses into carbon.

Life Cycle of a StarAll stars evolve the same way up to the red giant phase. The amount of mass a star has determines its life cycle from this point.

Life Cycle of a LargeStar

When stars that are much more massive than our Sun reach the red giant phase, their core temperature increases. Carbon atoms are formed from the fusion of helium atoms. Carbon fuses to form oxygen, oxygen to nitrogen, and finally nitrogen to iron. When the core contains iron, fusion stops because iron is the most compact and stable of all the elements. There is not enough energy to fuse iron.

Life Cycle of a LargeStar

Energy is no longer being generated in the core so gravity begins to win the battle. In less than a second, the star begins to collapse because of gravity. The iron atoms are crushed together and the temperature rises to 100 billion degrees. The repulsion between the iron atoms overcomes the force of gravity and recoils outward as a shock wave, which is called a supernova explosion. The supernova explosion provides enough energy to form many of the heavier elements in our universe. The material from the explosion is blasted out into space and is called a supernova remnant. A neutron star is left behind.

Life Cycle of a MediumStar

After the red giant stage, energy generation decreases and the star will collapse to a white dwarf star. White dwarfs are the final phase of all medium sized stars.

How light is generated and released from a star

Nuclear fusion combines two hydrogen atoms to form a helium atom releasing energy in the many forms including light and heat. It takes approximately 1 million years for a photon (light particle) formed in the core to reach the surface of the sun. It takes 8 minutes for the photon to reach Earth.

Different Types of Stars

The type of star usually depends on its mass.

Once fusion begins within a star, it is a main sequence star regardless of its size

Red Dwarf Stars

The most common type of star in our galaxy and probably the Universe. They live for very long periods of time (trillions of years) because they burn fuel slowly.

They are much smaller than our Sun, less than half the size. However, they can’t get too small or they wouldn’t be able to sustain nuclear fusion.

Most of the stars that you see in the night sky are not red dwarf stars. They are difficult to see because they are not very bright.

Yellow Stars

Yellow stars are medium sized stars like our Sun.

They are yellow because of the temperature at which they burn.

Yellow stars live for about 10 billion years and usually end their life as a white dwarf star.

Blue Giant Stars

Blue stars are very large, hot, dense stars that burn their fuel very quickly, only lasting abot 10,000 to 100,000 years before ending as a supernova explosion.

These stars are not very common, but because they are so bright, we can see them easily in the night sky.

Because they have such a short life span, astronomers use their location to search for areas of new star formation.

Bellatrix in the constellation Orion

Red Giant StarsRed giant stars occur after the main sequence when the hydrogen has been used up. Helium burns hotter than hydrogen and causes the star to expand great distances.

Red giants stars can end as white dwarfs, neutron stars, or black holes.

Blue giant stars, which are large to begin with, can turn into red supergiant stars before going supernova.

In about 5 billion years, our Sun will become a red giant star before exhausting its fuel and turning into a white dwarf.

White Dwarf StarsA white dwarf is a small, very dense, hot star that is made mostly of carbon.

These faint stars, or stellar remnants, are what remains after a red giant star loses its outer layers.

They are about the size of the Earth but much heavier.

They will eventually lose their heat and become a cold, dark black dwarf.

White dwarfs stars are often surrounded by a planetary nebula, a large shell of gases ejected after the end of a red giant star.

NGC 6326

Brown Dwarf StarsA brown dwarf is a "star" whose mass is too small to have nuclear fusion.

A brown dwarf is not very bright.

Therefore they are smaller than main sequence stars, more similar to the planet Jupiter in size, although much more massive.

Neutron StarsA neutron star is actually not a star but a stellar remnant. When a large star reaches the end of its life it undergoes a supernova explosion, leaving behind its very dense core.

A soup-can full of neutron star material would have about the same mass as our Moon. On average about 12 km in diameter and can be about 1.5 to 3 times the mass of our sun.

Neutron stars rotate very fast, sometimes several times per second. The fastest recorded is 716 times per second!

Black HolesA black hole is a region of spacetime from which gravity prevents anything, including light, from escaping.

It is a remnant of a supernova explosion of a very massive star.

The entire contents of the star are squashed down to a point smaller than a single atom, called the singularity. The singularity is actually thought to have zero volume and infinite density. The resulting gravity is immense. There is a certain point of no return around a black hole called the event horizon.

Black holes can only grow larger and may combine with other black holes to form supermassive black holes, which are believed to be at the center of most galaxies including our own Milky Way galaxy.

Comparison of Star Sizes

The Earth and our SunThe Earth has a diameter of 12,743km.The Sun is 4.6 billion years old and 109 times larger than the Earth with a diameter of 1,391,980 km.The Sun accounts for 99.86% of the mass of the Solar System.The Sun fuses 620 million tons of hydrogen every second.The Sun is brighter than 85% of the stars in the Milky Way galaxy.There are about 200-400 billion stars in the Milky Way galaxy. There are billions and billions of galaxies in the observable universe. There are approximately 30 billion trillion stars in the visible universe.

AldebaranA red giant star 68 million light years away in the constellation Taurus.

It has a diameter of 61 million km, about 44 times the size of the Sun.

One of the brightest stars in the nighttime sky.

BetelgeuseA 10 million year old red ‘Super’giant star located 640 light years away in the constellation Orion .

It has a diameter of about 862 million km. If placed at the center of our Solar system, it would engulf Mercury, Venus, Earth, and Mars, possibly reaching Jupiter.

VY Canis Majoris

A Red ‘Hyper’giant star in the constellation Canis Major.

It has a diameter that ranges from 1,975,000,000 km to 3,063,500,00 km. If placed at the center of our Solar System it could possibly engulf Saturn!

It would take 7 quadrillion Earths or 7 billion Suns to fill VY Canis Majoris.

Constellations‘Asterism’ is a term for a pattern of stars in the sky which is easily identifed and remembered.

In ancient times, people saw asterisms and associated them with mythological creatures and characters.

Astronomers began to make maps of the stars, and the named asterisms were included and called constellations.

ConstellationsConstellations are used to help identify positions of stars in the sky. They have imaginary boundaries formed by connecting the stars. All of the stars within or near a constellation are labeled with the name of that constellation.

Constellations are just patterns of stars as we see them from our viewpoint on Earth. They may look very different from a viewpoint on another planet.

While the individual stars in a constellation appear to be very close to each other, they can be separated by great distances in space and may have no actual connection to each other.

The time of year, time of day, and location on Earth determines which constellations you see in the night sky.

Orion the hunter

Taurus the bull

THE END