1. Hydrogen c. nitrogen neon d. oxygen
Review: Which of the following element is the main constituent of a star?
Hydrogen c. nitrogen
neon d. oxygen
Answer:
hydrogen

2. STARS: BIRTH & LIFE CYCLE
Looking at night sky:
How would you comment on the brightness of the stars (same? Varied?)
Any other comment about “looking at the night sky”: like looking back in time. Why?
What factors determine a star’s brightness? 1. its luminosity and 2. its distance from the observer on earth (how far away it is)
Some stars appear very bright but are actually fainter stars that lie closer to us.
Similarly, we can see stars that appear to be faint, but are intrinsically very bright ones lying far away from Earth. 

3. How do you describe how bright a star is?
2 factors:
The star’s brightness or luminosity and
Its distance from Earth
Luminosity
- a measure of the total amount of energy radiated by a star per second
Stars can be
10 000x less luminous than the Sun
or x more (e.g. Alpha Centauri A, Sirius, Vega)
Some stars far away but intrinsically very bright
Some stars because closer to us appear bright but intrinsically faint ones
AlphaCentauri A: 4.3 ly away from Sun; closest start to sun
Sirius 8.6 ly from earth: Vega (25 ly from sirius)
Proxima: closest star to earth
Why 32.6 ly reference?
A parsec is equivalent to 3.26 light years= the distance at which a star would have a parallax of one second of arc:
the basic formula relating the apparent (m) and absolute (M} magnitudes then is
M = m log D
where D is the distance to the object in pc.
Objects of the same luminosity that are located at different distances from us will have different apparent magnitudes.
It is the 'true' brightness — with the distance dependence factored out — that is of most interest to us as astronomers
astronomers calculate the brightness of stars as they would appear if it were 32.6 light-years, or 10 parsecs from Earth.
If gravitational waves exist, why bother to look for them?
LISA's goal is not limited to discovering low frequency gravitational waves, but to use them as a new window into astrophysical and physical phenomena that cannot be studied any other way. Gravitational waves carry information from objects that have no electromagnetic signature (such as the capture of neutron stars by massive black holes), whose electromagnetic signature is obscured by dust (GW are not absorbed) or is too weak (LISA detects amplitude, not power and can observe GW events out to redshifts of z~20)

4. Apparent magnitude "what you see is what you get" magnitude
the brightness of a star as seen from earth
NO consideration given to how distance influences the observation
The scale goes from -30 (the sun = -26) to +30 (Hubble space telescope = +29)
The more negative the number the brighter the star seen from earth

5. Absolute magnitude
 - "true" brightness, with the distance dependence factored out
Defined as the apparent magnitude that a star would have if it were (in our imagination) placed at 32.6 light years from the Earth
Sun = 4.7  Sun is not that bright compared to other stars
Different observers will come up with a different measurement, depending on their locations and distance from the star. Stars that are
closer to Earth, but fainter, could appear brighter than far more luminous ones that are far away.
Therefore, it is useful to establish a convention whereby we can compare two stars on the same footing, without variations in
brightness due to differing distances complicating the issue. To do so, we need to calculate the brightness of stars as they would appear if it were 32.6 lightyears,
or 10 parsecs from Earth.
Why 32.6 ly reference?
A parsec is equivalent to 3.26 light years= the distance at which a star would have a parallax of one second of arc:
So 32.5 ly = 10 parsec
the basic formula relating the apparent (m) and absolute (M} magnitudes then is
M = m log D
where D is the distance to the object in pc.
parallax

6. Colour and Temperature of Stars
A star’s colour can give us an idea of how hot that star is:
Blue  21,000-35,000C
Bluish-white
Yellow  our Sun is yellow (photosphere ~ 6,000C)
Orange
Red  3,300C
Color of star is a function of its temperature
Bluish-white  10, ,000 K
Which color indicates the hottest stars in the universe?

7. Hertzsprung-Russell (HR) Diagram
Turn to p343
Used to compare properties of stars
Shows luminosity versus temperature and colour
It is by convention reversed so that the hottest stars are located near the origin, and the coolest stars are to the right.
It was decided to standardize the brightness of each star so that it appeared to be located at a distance of 10 parsecs from the earth, or about 32.6 light years away. The brighntess of the sun would be set at "1", and other stars would be ranked accordingly.
Does that mean if we replot this HR diagram after the next 5 billion years it will look completely different? As the sun then will undergo a supernova and no longer in its main sequence as it is currently identified.
How long does main sequence last?

8. Main sequence The phase in which 90% of the stars are in
The phase in a star’s life cycle in which the process of nuclear fusion - hydrogen to helium – has stabilized. 
 seen as the diagonal band running from the top left to the bottom right on the Hertzsprung-Russell (HR) Diagram
Top left = hot, luminous, massive stars
Bottom right = cool, dim, low mass stars 

9. Who was right? http://www.youtube.com/watch?v=5az0W4Y1nuU

10. Life cycle of a star- A comparison

11. 3 categories of stars A star’s mass determines how it dies
1 solar mass = × 1030 kg
A star’s mass determines how it dies
Higher mass stars burn fuel faster and therefore die faster.
1. Low Mass Stars (or red dwarfs)
2. Medium Mass Stars:
3. High Mass Stars
0.5 solar mass or less
Consume hydrogen over 100 billion years
White dwarf
0.5 solar mass – 10 solar masses
e.g. The sun
Consume hydrogen in 10 billion years
Red giants
10 solar masses or larger
supernova
A star's mass is determined by the amount of matter that is available in its nebula, the giant cloud of gas and dust from which it was born. 
All stars evolve the same way up to the red giant phase
Neutron star if
between 10-40x the mass of sun
Black hole if
40x more than the mass of sun

12. Supernova Neutron stars the core made up of densely packed neutrons
The gradual build-up of heavy elements in the star’s centre causes the core to collapse sending out shockwave called a supernova
supernovae = star explosions
Releases many heavy elements which can help form new stars, planets, or other bodies
The elements in your body were created (fused together) in the cores of old stars!
As the star rips apart, a nebula is formed
Recall: what is a nebula?
the core made up of densely packed neutrons
The densest material known.
Found in centre of the Crab nebula
A star might exist for millions or billions of years then suddenly come to an end in few minutes
Supernovae
Neutron stars are formed by stars with masses greater than eight times the mass of our sun. In these stars there is enough fuel to produce larger quantities of carbon and oxygen. If the carbon and oxygen core has a mass greater than 1.4 times the mass of our Sun, the gravitational forces are strong enough to collapse the core beyond the white dwarf stage. The carbon and oxygen will fuse to produce neon, sodium and magnesium.
All of these fusion processes have emitted energy to keep the star burning. But the silicon and sulphur in the core produce iron when they fuse together. Iron is the most stable form of nuclear matter, and the fusion of iron does not emit energy. In fact, iron requires energy for fusion to take place. The result is that fusion stops at the very centre of the star.
With no radiation from the core, the outer layers of the star begin to collapse in towards the centre, drawn by a gravitational attraction. The iron core is pushed together so tightly that nuclei of iron begin to touch, before emitting an immense shockwave.
This shockwave of very high energy particles spreads outwards through the star and holds enough energy to fuse elements together into isotopes of every imaginable element, including very heavy substances like uranium. The shockwave also spreads inwards through the core with enough energy to convert the protons and electrons of the iron into neutrons. The explosion is so powerful that the supernova will outshine the rest of the galaxy for a month.
Life cycle of a star recap:

14. Learning checkpoint
Choose from the following hypotheses regarding length of star life:
 1)  The bigger a star is, the longer it will live.  2)  The smaller a star is, the longer it will live.
Answer:
The smaller a star is, the longer it will live.
Because larger stars burn fuel faster than smaller stars
Interactive game:
Larger stars have more fuel, but they have to burn (fuse) it faster in order to maintain equilibrium. Because thermonuclear fusion occurs at a faster rate in massive stars, large stars use all of their fuel in a shorter length of time. This means that bigger is not better with respect to how long a star will live. A smaller star has less fuel, but its rate of fusion is not as fast. Therefore, smaller stars live longer than larger stars because their rate of fuel consumption is not as rapid.

15. Discovered by Bell and Hewish (1968)
Pulsars
- Pulsar: a celestial object, thought to be a rapidly rotating neutron stars , that emits regular pulses of radio waves that can be detected on Earth
Discovered by Bell and Hewish (1968)
Neutron stars: They are mostly made of neutrons that formed as electrons combined with protons in the atomic nuclei of the dying stars' collapsing cores.
Neutron stars have powerful magnetic fields that can be detected as radio pulses on the Earth each time they rotate. These objects are known as pulsars when the pulses can be detected on the Earth
Pulsars were discovered by accident in 1967 while Jocelyn Bell and Antony Hewish were looking for twinkling sources of radio radiation. The explanation for the radio pulses proved the existence of neutron stars, incredibly dense remains of massive collapsed stars.
Enter this section to read about the discovery, and find out how a dying star can become a pulsar.
Neutron stars : incredibly dense remains of massive collapsed stars
Pulsars = pulsing stars