1. Review the Sun

2. Review the Sun

3. Hydrogen neon c. nitrogen d. oxygen
Review: Which of the following gases is found in the core of a star like our Sun?
Hydrogen
neon
c. nitrogen
d. oxygen
Answer:
a- hydrogen

4. THE STARS 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. 

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

6. How do you describe how bright a star is?
2 factors:
The star’s brightness or luminosity and
Its distance from Earth
Luminosity = Star brightness
is how much light energy a star gives off per second
we assign a luminosity of 1.0 to our Sun
if a star has a luminosity of 3.0, it gives off three times the light energy of the Sun
Alpha Centauri A, Sirius, Vega are stars ~ x brighter than the Sun
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 star to sun
Sirius 8.6 ly from earth: Vega (25 ly from sirius)
Proxima: closest star to earth, similar to our Sun
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)

8. Apparent magnitude
"what you see is what you get" magnitude
the brightness of a star as seen from earth
Determined by how luminous the star is and how far away it is
Stars that are closer to Earth, but fainter, could appear brighter than far more luminous ones that are far away.
The more negative the number the brighter the star seen from Earth (e.g. Sun is -26)

9. Practice apparent magnitude
Which star is the brightest?
The more negative the number the brighter the star seen from Earth
Therefore Star 3
Stars
Apparent magnitude
Star 1
+ 0.8
Star 2
+ 2
Star 3
- 3
Star 4
+ 6

10. - a star’s "true" brightness
Absolute magnitude
 - a star’s "true" brightness
How bright a star is if it were 33 light year from Earth
- the influence of distance on the star’s brightness is factored out
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.

11. 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?

12. Hertzsprung-Russell (HR) Diagram

13. Hertzsprung-Russell (HR) Diagram (p310 txtbook)
Used to compare properties of stars
This diagram shows luminosity versus temperature and star colour
How bright a star is also linked to its surface temperature
From the HR diagram,
1. Which star is the hottest among
Alpha Centauri B, our Sun, Rigel and Betelgeuse?
Answer: Rigel
2. Which star is the brightest among
Vega, Sirius B, our Sun and Betelgeuse?
Answer: Betelgeuse
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?

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

15. https://www.youtube.com/watch?v=12C4t4Gs3Bw (to 8:40min)
Today’s joke
The visible light from a distant star will appear to twinkle when viewed from earth, because the light has to pass through the earth's atmosphere to reach us. The atmosphere consists of many moving layers of air at different temperatures and densities. Light refracts differently through each layer of atmosphere, so the star appears to be wiggling slightly when seen from the ground. This produces the twinkling, or scintillation.
(to 8:40min)

17. Answers to text questions #1-7 on page 311
Other units were too short. Light Years were the largest unit of measuring distance.
Absolute magnitude is more effective for measuring a star’s actual brightness.
Sirius (-1.5) is brighter than Deneb (+1.3).
If a star is closer to the Earth, it will appear brighter. For example….
Our sun is hotter than Alpha Centauri B; Vega is both warmer and bigger than the Sun