Amanda is a retired educator with many years of experience teaching children of all ages and abilities in a wide range of contexts.

1. What Are Stars?

Stars are huge spheres of burning hydrogen gas with immense nuclear reactions taking place in their centers. The force of gravity keeps their particles together and stops the stars from exploding. When a star is first born it creates energy by fusing atoms of hydrogen together to create helium.

A Star is Born

2. Three Kinds of Star Groups

Unlike the sun, it’s uncommon for stars to exist on their own. Most cluster together in systems of two or more stars. In the constellation of Orion, three stars compose the Mintaka cluster. In Gemini, Castor boasts six stars. Stars cluster in connected groups formed from nebulae. Bound by gravitational forces, up to 60% of all stars stay in their groups. Single stars, such as our Sun, are rare.

There are three kinds of star groups:

• binary stars
• eclipsing binary stars
• variable stars

Binary Stars

Binary stars have equal mass and density and orbit around a common gravitational center.

Eclipsing Binary Stars

When you see a star in the night sky which seems to "twinkle", what you are really observing is an eclipsing binary group. These are two stars of unequal size. The smaller star orbits the larger one, regularly "eclipsing" its light from view. From Earth, that makes the star appear to twinkle.

Variable Stars

As the name suggests, variable stars have fluctuating brightness. Sometimes massive explosions in their surfaces cause them to brighten. At other times, when the star is less reactive, it will seem to dim.

3. Record-Breaking Stars

The Faintest Star

Astronomers call the faintest star known to us, RG 0058.8-2807. It's a brown star one million times less bright than the sun.

The Brightest Star

The brightest star known to science was a supernova recorded in the Anglo-Saxon Chronicles in the 11th century! Astronomers now know it to have been SN 1006 which flared so brightly it was visible during the day.

The Fastest Star

The fastest star is a pulsar called PSR 1937+214 which rotates at a speed of 642 times a second.

4. The Life-Cycle of a Star

Every star starts out as a giant cloud of gas and dust particles. When gravity causes the cloud of dust and gas to implode, it releases huge amounts of energy and the star begins to shine. Most stars survive for billions of years. A smaller star, such as our sun, eventually swells to become a red giant. A red giant may have a diameter of 100 times the diameter of the sun. Larger stars may become supernovas, releasing more energy in a single minute than our sun radiates over 9 billion years.

The Seven Stages of the Stellar Life-Cycle

• a huge molecular cloud of dust and gas implodes becoming dense and energetic
• sections of the molecular cloud contract further to become proto-stars. Proto-stars become very dense and very hot. As they spin the proto-stars flatten into a disc like shape
• the gases and molecular particles in the proto-stars cause nuclear reactions, creating violent stellar winds as gravity draws any remaining particles together to form planets which orbit the new star
• once a star has formed it radiates energy, making it shine. Smaller stars are longer lived and larger stars have shorter lifetimes because they burn hydrogen faster
• once a star uses up its main supply of hydrogen, it fuses helium into carbon causing its outer layers to expand and glow red
• the star has now become a red giant, its intense heat expanding and destroying the surrounding planets as its core fuses carbon into iron and collapses under its own weight
• the final stage of the star's life is a massive explosion called a supernova in which the star burns as bright as a billion suns and, at last, explodes

5. The Six Kinds of Stars

There are six kinds of stars. The mass of the star determines its brightness, its color, the temperature at its surface, its overall size, and its lifespan. Our sun is a yellow star of average size and temperature. Larger stars produce hotter surface temperatures.

1. the smallest kind of star is a brown dwarf with a surface temperature of 1,800°F
2. a red dwarf is the next largest, with a surface temperature of 5,100°F
3. a yellow star, such as our sun, has a surface temperature of 9,900°F
4. the next largest is a white star with a surface temperature of 18,000°F
5. then comes a blue/white star with a surface temperature of 28,800°F
6. a blue star, the biggest, has a surface temperature of 43,200°F

Each star begins and ends life in the same way, but its "main sequence" varies depending on its mass.

6. Our Closest Stars

7. The Earliest Recorded Supernova

Ancient Chinese astronomers observed the earliest recorded supernova, the remains of a dying star, in the 11th century. With a powerful telescope, you can see its last remaining molecular particles in the Crab nebula. The nebula is expanding at almost 1000 mi/s (miles per second).

8. The Brightest Stars You Can See Without a Telescope

A Black Hole

9. What Happens After a Star Dies?

When a star reaches the end of its life-cycle either as an explosive supernova or a planetary nebula, it collapses into one of three forms:

• a white dwarf

if the remaining matter after a star dies has less than one-and-a-half times the mass of the sun, it becomes a white dwarf. White dwarfs are the super-dense cores left over after the remains of a typical planetary nebula disperses into space

• a neutron star

when a supernova leaves a residual mass of between one-and-a-half and three times that of the sun it collapses into the densest form of matter, known as a neutron star. Neutron stars are the densest objects in the universe. A particle of a neutron star smaller even than a pinhead would weigh over 1 million metric tonnes. Some neutron stars, known as pulsars, rotate. They generate intense magnetic fields sending out radiation beams far across the universe

• a black hole

a black hole is an area of potentially infinite gravity around a point of infinite density known as a singularity. Not even light can escape if it falls beyond the edge of a black hole. Astronomers call the edge of a black hole, the “event horizon”. Black holes occur when giant supernovas of over three times the sun's mass collapse in on themselves.

A Sky Full of Stars

10. How Many Stars in the Universe?

How many stars are there in the universe? The short answer is, nobody knows. The universe is just too big and we can only study a small part of it known as "the observable universe". Beyond that, we know nothing at all.

An average galaxy may contain 100 billion stars and it would take over a thousand years to count them all at a rate of about three per second. The observable universe has hundreds of thousands of such galaxies. So, while we cannot put a final figure on the number of stars in the universe, we do know that it must be many billions of billions.

Mind blowing, isn't it?

© 2018 Amanda Littlejohn

Comments

Amanda Littlejohn (author) on September 18, 2018:

Hi Shelley,

Thanks for your comment. The stars are fascinating, aren't they?

FlourishAnyway from USA on September 18, 2018:

Good information here, Amanda. I learned many things from reading!