Stars

Distances To The Stars Stars are separated by

vast distances. Astronomers use units

called light years to measure the distance between the stars

A light-year is the distance that light travels in the vacuum during a year

Proxima Centauri, is the closest star to the sun.

Parallax Astronomers have developed various methods of

determining the distance between the stars. The change in the position of an object with respect to

a distant background is called parallax. As the Earth moves on its orbit, astronomers are able

to observe stars from two different positions. Astronomers measure the parallax of nearby stars to

determine their distance from the Earth

Properties of the Stars Astronomers classify stars by

their colour, size, and brightness. Other properties of the stars are their chemical composition and mass.

Colour and Temperature – a star’s colour indicates the temperature of its surface.

The hottest stars appear blue The cooler stars appear red The spectrum of colour in a

star is from blue to green, to yellow and red.

Brightness The brightness of a star as

viewed from the Earth is dependent on many factors such as colour intensity and distance.

Apparent Brightness – is the brightness of a star as it appears from the Earth.

The apparent brightness decreases as its distance from you increases

Absolute Brightness – is how bright a star really is. The absolute brightness is a characteristic of the star and is not dependent on its distance from the Earth.

Composition Each star has its

own spectrum. Most stars have a

chemical makeup that is similar to the sun, with hydrogen and helium together making up from 96 to 99.9 % of a star’s mass.

How Stars Are Formed A nebula is a large cloud

of gas and dust spread out over a large volume of space.

Some nebulas are glowing clouds lit from within by bright stars.

A star is formed when a contacting cloud of gas and dust becomes so dense and hot that nuclear fusion begins.

Life Cycles of the Stars Adult Stars – A star’s

mass determines the star’s place on the main sequence and how long it will stay.

The amount of gas and dust available when a star is formed deter-mines the mass of each young star.

The larger the star the more energy it produces.

Since blue stars burn brightly, they use up their fuel quickly and are short lived.

The Death of a Star The dwindling

supply of fuel in a star’s core leads to the star’s death as a white dwarf, neutron star, or black hole.

Groups of Stars Astronomers have

determined that more than half of all stars are members of the star systems.

There are three basic kinds of star clusters: open clusters, associations, and globular clusters.

Astronomers classify galaxies into four main types: spiral, barred-spiral, elliptical, and irregular.

The Expanding Universe The observed red shift in the spectra of the

galaxies shows that the universe is expanding. Astronomers theorize that the universe came

into being in an event called the Big Bang. Dark matter can only be detected by observing

its gravitational effects on visible matter.

Pavel Elena-Maria, Sav Ioana, Ispas DariusXC-matematica-informaticaColegiul National “Horea, Closca si Crisan” Alba Iulia