Download Stars I

A105
Stars and Galaxies
Today’s APOD
 News Quiz Today
Jewelbox homework due Thursday
Announcements…
• Kirkwood Obs. open Weds night
8:30-10:30 PM
• Rooftop Sessions, Oct. 10 & 11,
9:00 PM
• Remote Obs Oct. 14 @ 10 PM
and Oct. 16 @ 7 AM
The Sun Today
• Image credit: Solar
Orbiting
Heliospheric
Observatory/MDI
Not very interesting
today!
Stars
 Basic Properties of Stars
 distance
 brightness
 diameters
 The Hertzsprung-Russell Diagram
The Brightness of Stars
• Apparent brightness – how bright does it
look in the sky?
• Absolute brightness – how bright is it
really??
• The apparent brightness depends on both a
star’s distance and its intrinsic brightness
The Inverse Square Law tells us how a
star’s apparent brightness changes
with distance
• Brightness decreases
as distance squared
– something twice as far
away will be four times
fainter
– something 10 times
further away will be 100
times fainter
– something 1000 times
further away will be a
million times fainter
How Far Away Are Stars?
If we know a star’s apparent AND absolute
brightness, we can calculate its distance
brightness changes as 1/distance2
The inverse square law describes how
the brightness of a source light (a star!)
diminishes with distance
But how do we get the distances to stars
whose brightness we DON’T know?
Measuring the
distances to
stars using
Parallax
Measuring the distances of stars
Parallax : apparent change in the
position of an object due to a change in
the position of the observer
Stellar parallax uses the Earth’s orbit
as the baseline
Parallax
What is a Parsec???
Parsec: the distance to an object
with a stellar parallax of one arc second
A star at a distance of 1 parsec shows
a parallax of 1 arc second
1 parsec = 3.26 light years
A parallax of ~0.001 arc seconds
is the smallest we can measure
How big is one
arc second?
The size of a
dime at a
distance of
2.3 miles!
The parallax of Alpha Centauri = 0.76 arcseconds
How Big Are Stars?
We can’t see the stars’
diameters through a telescope.
Stars are so far away that we
see them just as points of light.
If we know a star’s temperature and its luminosity,
we can calculate its diameter.
How do we determine a star’s
temperature?
Luminosity depends on….
TEMPERATURE the hotter a star is,
the brighter it is.
DIAMETER –
the bigger a star is,
the brighter it is.
Stars range in size from about the size of the Earth to
hundreds of times the Sun’s diameter
Magnitudes
• Astronomers use “magnitudes” to describe how
bright stars are
• Small numbers are brighter, large numbers
fainter.
• The brightest naked-eye stars are around
magnitude zero.
• The faintest naked-eye stars are around
magnitude six
• 5 magnitudes are a factor of 100 in brightness (a
6th magnitude star is 100 times fainter than a 1st
magnitude star)
But only
certain sizes
and colors
are allowed!
Stars come in many sizes and colors
HR Diagram Simulator
Key Ideas – The HR Diagram
• The intrinsic brightness or luminosity of
stars depends on temperature and radius
• if two stars have the same radius, the hotter
one is brighter
• if two stars have the same temperature, the
bigger one is brighter
• The Hertzsprung-Russell Diagram
• relates the temperature and brightness of
stars
The Main Sequence
BRIGHTNESS
The sun is an
ordinary,
yellow main
sequence star
TEMPERATURE
Giants and Supergiants are
cooler and very large
Supergiants
BRIGHTNESS
Giants
White dwarfs
are small and
hotter
TEMPERATURE
Most stars occur in these main
groups in the luminositytemperature diagram
BRIGHTNESS
 Main Sequence
 Giants
 Supergiants
 White Dwarfs
TEMPERATURE
Quiz: Which star is the biggest?
BRIGHTNESS
A
B
C
D
TEMPERATURE
Quiz: Which star is the
smallest?
BRIGHTNESS
A
B
C
D
TEMPERATURE
The Nearest and the Brightest
Goal:
– to learn about types of stars
– to explore the stars near the Sun and
compare them to the stars we see in the sky
Task:
– plot a Hertzsprung-Russell diagram including
both the nearest stars and the brightest stars
in the northern sky
Familiar Stars
1000 ly
A little farther out
The
Brightest
Stars in the
Sky
(no need to copy
these down!)
Star
Distance
(LY)
Temperature
(K)
Absolute
Magnitude
Sun
0.000015
5800
4.8
9
9600
1.4
232
7600
-2.5
Alpha Cen A
4
5800
4.4
Arcturus
37
4700
0.2
Vega
25
9900
0.6
Capella
42
5700
0.4
Rigel
773
11000
-8.1
Procyon
11
6600
2.6
Achernar
144
22000
-1.3
Betelgeuse
427
3300
-7.2
Hadar
335
25000
-4.4
Acrux
321
26000
-4.6
Altair
17
8100
2.3
Aldebaran
65
4100
-0.3
Antares
604
3300
-5.2
Spica
263
2600
-3.2
Pollux
34
4900
0.7
Sirius
Canopus
Hertzsprung Russell Diagram - Brightest Stars
-10
-5
Absolute Magnitude
Plot Absolute
Magnitude vs.
Temperature
0
5
10
15
20
30000
25000
20000
15000
10000
Temperature (K)
5000
0
The Nearest Stars
Distance
(LY)
Temperature
Absolute
Magnitude
Prox Cen
4
2800
15.53
Alp Cen A
4
5800
4.4
Alp Cen B
4
4900
5.72
Barnard’s
6
2800
13.23
Wolf 359
7.5
2700
16.57
Lal 21185
8
3300
10.46
Sirius A
9
9900
1.45
Sirius B
9
12000
11.34
Luyten 726-8A
9
2700
15.42
UV Ceti
9
2600
15.38
Ross 154
10
3000
13.14
Star
The Nearest Stars
Hertzsprung Russell Diagram
-5
Absolute Magnitude
Adding
the
Nearest
Stars to
the HR
Diagram
-10
0
5
10
15
20
30000
25000
20000
15000
10000
Temperature (K)
5000
0
Hertzsprung Russell Diagram
-10
-5
Absolute Magnitude
The HR
Diagram
Giants and
Supergiants
0
5
Main
Sequence
10
White
Dwarf
15
20
30000
25000
20000
15000
10000
Temperature (K)
5000
0
Key Ideas – Stellar Census
• Comparison of Main Sequence,
Giant, and White Dwarf Stars
• The Family of Stars
• What are the most/least common kinds
of stars?
• Why are red dwarfs so common?
The brightness of a star depends on
distance, luminosity, and temperature
Most luminous
stars:
106 LSun
Least luminous
stars:
10-4 LSun
(LSun is
luminosity of
Sun)
Most massive
stars:
100 MSun
Least massive
stars:
0.08 MSun
(MSun is the
mass of the
Sun)
Main-Sequence Star Summary
High Mass:
High Luminosity
Short-Lived
Large Radius
Blue
Low Mass:
Low Luminosity
Long-Lived
Small Radius
Red
Stellar Properties Review
Luminosity: from brightness and distance
10-4 LSun - 106 LSun
Temperature: from color and spectrum
3,000 K - 50,000 K
Mass: from binary-star orbits
0.08 MSun - 100 MSun
Constructing an HR Diagram
Apparent Magnitude
0
5
10
15
-0.5
0
0.5
B-V Color
1
1.5
2
What’s this B-V color?
• Astronomers measure the brightness of stars in
different colors
– Brightness measured in blue light is called “B” (for
“Blue”)
– Brightness measured in yellow light is called “V” (for
“Visual)
• Astronomers quantify the “color” of a star by
using the difference in brightness between the
brightness in the B and V spectral regions
• The B-V color is related to the slope of the
spectrum
The slope of the spectrum is different at
different temperatures
 Homework #7 Due THURS.
 Events this week
 Rooftop Oct 10 & 11
 Kirkwood Open Night Oct. 11
 Remote Observing Oct 14 & 16