The Universe
- Stars

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Two factors are obvious about stars

  1. – due to temperature

We measure brightness as a measure of

Generally, magnitude is measured on a scale originally from 1 to 6 with
Since the scale was devised some objects fall outside these ranges and very bright objects can have values in the

The scale is logarithmic meaning

For example a magnitude 5 star is brighter than a magnitude 6 star.

Similarly a magnitude 4 star is 2.5 times brighter than a magnitude 5 star but is times brighter than a magnitude 6 star.

magApparent magnitude is how Obviously, if all stars had the same brightness, the ones closer to us would appear brighter.  In actuality, bright stars might be close to us or not and this will mean they appear brighter or dimmer based on how far away they are.  This is the apparent magnitude.

orionFor example and Bellatrix in the

Betelgeuse has an apparent magnitude of 0.6 and Bellatrix has an apparent magnitude of 1.6. Since Betelgeuse has a magnitude about 1 x greater than Bellatrix it appears about .

But which is closer?

The Distances Between Stars

The kilometer is too small to measure the immense distances in space. Often we use (l.y.), which is the distance light travels in one year. Since light travels at So in 1 year light will travel about 9.5 x 1014 kilometers or .

Betelgeuse is . away!!!!!

Another measure used in space is the (pc) a parsec is

Betelgeuse is away!

appThe Parsec is based on the idea of parallax. Parallax is the .


The to us the more parallax we notice.

Thus, close stars appear to move in the sky as we orbit the sunparr

Even close stars only move less than 1/1000 th of a degree.

Absolute magnitude

This is a measure of how bright a star would appear to us if it were .
If Betelgeuse was 10 pc (it is actually about 200pc away) from Earth it would have a magnitude of

. This is it would be than we see it currently. Bellatrix on the other hand would

have a magnitude of at 10 pc. (Remember the small the number the brighter the star)


< This is only a small part of the Electromagnetic (EM) spectrum. But it is how we see stars.

The colours can range from through to (like a rainbow).

Although stars emit light from the red side to the blue side they have colour areas from which they .

For example, cooler stars emit more than hotter stars that emit more .

Colour is related to .

Another device used to analyse stars is a (which works to spilt light into its component colours – like a rainbow)

It is possible to determine what are present in a star by the use of a spectrometer.

Eg an of the sun


Notice the lines indicate the presence of and

From the different stars scientists have a classification of stars by spectral classesSpectral Class

Nuclear Fusion

Because there is a lot of matter (mass) in a star, there is a . Most of a normal star is made up of hydrogen.

As the gravity of the star it . This causes hydrogen atoms to (join) together to form and some is given off.

This energy is the energy we get from the sun


The Life Cycle of Stars

HRHertzsprung-Russell diagrams

A system for classifying stars usually using and

Stars tend to move in this diagram over periods of time as they

Main sequence

The band of stars stretching from the top right to the bottom left of the HR diagram are called The stars in the top left are and temperatures whereas those in the bottom left are and temperature.

As stars age they can get (ie move from top right to bottom left), thus they may move places in the HR diagram.  This happens after of years.

Red Giants


A main sequence star cools as it gets older.  As it cools it in on itself (due to gravity) the gases that are left on the outside of the star start to and as they do they and as they expand they .

So we have a cool star () expanding (making it ) --> .

In only about the red giant will continue to collapse and the outer layers will escape as a cloud of gas called a .  The core gets very hot and emits light creating a spectacular celestial sight.


Super Giants

Stars bigger than our sun start in the main sequence but because they are so big they use up their energy much quicker. But due to their size they don’t as quickly and they move slowly from in the HR diagram. As they run out of energy eventually they undergo a spectacular change called a


These come about because most of the material in the inner core has been fused into iron (it cannot fuse into other elements) and it becomes very massive pulling more and more to the middle.  As the iron collapses it collides producing a called a supernova.

CrabThe is an amazing example of a supernova.
What’s left can (if mass of material left is the mass of our sun) form a .  It’s called a neutron star because and fuse together to form .

Black Holes

If the mass left after a supernova is greater than our sun, the matter ends up compacting together so tightly that not even can escape from its gravitational pull. This is called .

They are very hard to detect because they do not emit any . Two indirect ways are:

  1. The effects on a close star in a arrangement.BHole1
  2. The of light passing from a star behind the black hole.


A summary of star life cycles is such