Astronomy 192401

1
Astronomy 192-401 Stars, Galaxies, and the
Universe
Well be starting with a quiz today. There will
be five questions. You can get one wrong without
being penalized. In other words, the quiz will
scored as follows Number correct Grade 4-5 10
0 3 75 2 50 1 25 0 0 Please take
out a sheet of paper and put your name and
todays date on it. Remove all other papers and
books from the top of your desk.
2
Quiz 5
1) If you double your distance from a star, the
apparent brightness will a) double b)
quadruple c) stay the same d) halve e) quarter 2)
True or False All stars on the Main Sequence
have roughly the same luminosity. 3) The two
types of star clusters are Open Clusters and
___________ Clusters. 4) True or False In a
photograph (taken by a telescope) showing many
galaxies as well as stars, you can tell the stars
from the galaxies because the stars will appear
to have spikes. 5) True or False White Dwarves
produce energy via fusion.
3
Reminder
The first test will be Wednesday, Feb. 20. The
test will include all material up to, and
including, Chapter 14 (Our Star). There will
be a review session during class on Monday, Feb.
17.
4
Reading Assignment
Read Chapter 16 (Star Birth). Ensure that
you know the items listed in the Summary Of
Key Concepts. Expect a quiz on Chapter 16 on
Monday (Feb. 25)!!
5
Announcement
Lecture notes (for lectures Ive presented) are
now available from the class website. Note that
the files are somewhat large.
6
(No Transcript)
7
Parallax
Astronomers dont usually use light years they
prefer parsecs.
parsec (pc) the distance to an object with a
parallax angle of 1 arcsecond.
1 parsec 3.26 light years
Parallax is the only method of measuring stellar
distances without making assumptions about the
nature of stars.
8
Luminosity
Luminosity 4 p d2 Brightness
Now that we know the distance, we can determine
the luminosity.
Proxima Centauri Luminosity 0.0006 LSun
Betelgeuse Luminosity 38,000 LSun
Stellar luminosities range from 10-4 LSun to 106
LSun
9
Spectral Type
A way of classifying stars based on their
spectral properties.
Oh Hottest Be A Fine Girl The Suns spectral
type Kiss Me Coolest
10
Magnitude System
Originally classified stars by how bright they
appeared to the human eye.
Scale ran from 1 (the brightest) to 6 (barely
visible)
Modern magnitude scale A difference of 5 means
the apparent brightness changes by 100.
A difference of 1 means the apparent brightness
changes by a factor of 2.51.
Absolute Magnitude the apparent magnitude a
star would have if it were 10 pc (32.6 ly) away.
11
Magnitude System
12
Magnitude System
13
Determining the mass of a star
How do we know the mass of the sun?
Newtons version of Keplers Third Law
Where p orbital period a average distance
between centers G gravitational constant M
mass
14
Determining the mass of a star
15
Determining the mass of a star
Can we use this method for other stars?
Need to have two objects orbiting each other
Fortunately there are lots of binary star systems.
16
Types of Binary Star Systems
Visual Binary Can see (at least one of) the
stars orbiting each other.
Eclipsing Binary Brightness changes. When
neither star is eclipsed, we see the combined
light of both stars. When one star eclipses the
other, the apparent brightness drops.
Spectroscopic Binary Detect the orbital motion
of (at least one of) the stars by observing
Doppler shifts in spectral lines.
17
Determining the mass of a star
We need to know the orbital period, and the
distance between the stars.
Orbital period is easy observe it directly for
visual binaries. time between eclipses for
eclipsing binaries. time between the same
doppler shifts for spectroscopic binaries.
The distance between the stars is more difficult
but if we know the velocities and the period,
we can find the distance.
18
Categorizing Stars
Two important criteria Temperature Luminosity
What happens when you make a scatter plot of
stellar temperatures and luminosities?
19
You WILL get tired of looking at it by the end of
class!
Hertzsprung-Russell Diagram
20
Hertzsprung-Russell Diagram
Ejnar Hertzsprung Credit Dorritt Hoffleit, Yale
University Observatory, courtesy AIP Emilio
Segre Visual Archives
Henry Norris Russell Credit AIP Emilio Segre
Visual Archives, Margaret Russell Edmondson
Collection
21
Hertzsprung-Russell Diagram
Most of the work was done by computers.
How many computers do you see in this picture?
22
Hertzsprung-Russell Diagram
What is THE MOST IMPORTANT thing about the H-R
Diagram?
The distribution of stars is NOT random.
Most stars fall in a narrow band called the Main
Sequence.
23
(No Transcript)
24
High mass
High L
Main Sequence stars fuse hydrogen to produce
energy.
Low mass
Why is there a link between mass and luminosity
for main sequence stars?
Low L
25
Hertzsprung-Russell Diagram
For Main Sequence stars, mass is the most
important attribute.
26
High mass
High L
Lifetimes increase along the main sequence why?
Low mass
Low L
27
Main Sequence Lifetimes
Star Mass Luminosity Lifetime 61 Cygni
B 0.5 0.1 Sun 1 1 1010 Achernar 6 1,000
5 x 1010
6 x 107
Does this agree with the H-R Diagram?
28
5 x 1010 ?
6 x 107 ?
29
Stellar Radii
How can we determine a stars size?
Stefan-Boltzmann Law emitted power per unit
area s T4
30
Factor of 1011
Size increases along the Main Sequence
Factor of 10
31
Giants, Supergiants, White Dwarves
Giants and Supergiants Low Temperature High
Luminosity Big
White Dwarves Remnant cores that are cooling
Giants, Supergiants, and White Dwarves all
started as Main Sequence Stars.