Transcript about Stars

H205
Cosmic Origins
APOD
Properties of Stars (Ch. 15)
The Milky Way (Ch. 19)
EP3 Due Wednesday
Opportunities
• Kirkwood Obs. Open April 1 (weather permitting)
• Solar Telescope open April 4 (weather permitting)
• Astronomy Club – Mondays at 7:30, Swain West 113
(Includes PIZZA!)
• Remote Observing – April 9 & 10, 9:30-midnight, Swain
West 403
• PBS on March 31 (Check times!):
– Investigation into a possible comet strike
• PBS on April 21?
– 8 PM: 400 Years of the Telescope - narrated by Neil deGrasse
Tyson
– 9 PM: - A Sidewalk Astronomer - the story of John Dobson
(now 91 years old!)
 Basic Properties of Stars
 distance
 brightness
 diameters
 The Hertzsprung-Russell Diagram
Stars
The Brightness of Stars
• Apparent brightness – how bright does it look in
the sky?
Apparent magnitude - mV
• Absolute brightness – how bright is it really??
Absolute magnitude - Mv
• 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
Apparent Brightness 
Luminosity
4π distance 2
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
1 AU
 sin (p)
distance
Parallax
A parsec is the
distance at which
1 AU subtends an
angle of 1 arc sec
1 AU
distance
Angle = p
1
 distance (in parsecs)
p ( in arcsecs)
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.
Stellar Radii
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, apparent magnitude,
and distance, we can calculate its radius
Temperature from
Luminosity from parallax and
apparent magnitude
Luminosity  4R T
2
4
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)
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
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
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
-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 – 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
But only
certain sizes
and colors are
allowed!
Stars come in many sizes and colors
HR Diagram Simulator
BRIGHTNESS
Most stars occur in these main
groups in the luminositytemperature diagram
TEMPERATURE
Main
Sequence
Giants
Supergiants
White
Dwarfs
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
The apparent brightness of a star in the sky 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
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
Most stars fall
somewhere
on the main
sequence of
the H-R
diagram
WHY
WHY
WHY
???
High-mass stars
Low-mass stars
Mass
measurements
of mainsequence stars
in binary star
systems show
that the hot,
blue stars are
much more
massive than
the cool, red
ones
Main-sequence
stars are fusing
hydrogen into
helium in their
cores like the Sun
massive mainsequence stars are
hot (blue)
and luminous
Less massive stars
are cooler (yellow
or red)
and fainter
High-mass stars
Low-mass stars
The mass of a
normal,
hydrogenburning star
determines its
luminosity and
temperature!
Mass & Lifetime
Sun’s life expectancy: 10 billion years
Life expectancy of 10 MSun star:
10 times as much fuel, uses it 104 times as fast
10 million years ~ 10 billion years x 10 / 104
Life expectancy of 0.1 MSun star:
0.1 times as much fuel, uses it 0.01 times as fast
100 billion years ~ 10 billion years x 0.1 / 0.01
Off the Main Sequence
• Stellar properties depend on both mass and
age: those that have finished fusing H to He in
their cores are no longer on the main sequence
• All stars become larger and redder (and cooler)
after exhausting their core hydrogen: giants and
supergiants
• Most stars eventually end up small and hot after
fusion has ceased: white dwarfs
Star Clusters
Star
Clusters
Goals:
– Understand how we learn about stellar evolution
from the properties of stars in clusters
– Understand how we can determine the distances
of star clusters
– Understand how we can determine the ages of
star clusters
“Globular" Clusters and “Open" Clusters
Globular Clusters
Open Clusters
•104-106 stars
•old!
•compact balls of stars
•high star density
•10-104 stars
•generally young
•loose
•low star density
Properties of Stars in Clusters
• Formed at the
same time
• Stars are the same
age
• All stars have the
same composition
• The stars are held
in a group by their
common gravity
Cluster HR Diagrams
Hotter stars are brighter in
blue light than in yellow
light, and have low values
of B-V color, and are found
on the left side of the
diagram.
Cooler stars are brighter in
yellow light than in blue
light, have larger values of
B-V color, and are found on
the right side of the
diagram.
hotter
cooler
Distances to Star Clusters
The Sun has a “B-V”
color of about 0.6.
Stars like the SUN
What would the apparent
magnitude of the Sun be
at the distance of the
cluster Messier 6?
Stars in Messier 6 with
B-V colors of 0.6 have
similar mass and
luminosity to the Sun
hotter
cooler
Ages of Star Clusters
The “bluest” stars left
on the main sequence
of the cluster tell us the
cluster’s age.
As the cluster ages,
the bluest stars run out
of hydrogen for fusion
and lose their “shine”
hotter
cooler
Jewelbox
The HR diagrams of
clusters of different
ages look very
different
5
10
M 67
0
15
-0.5
0
0.5
B-V Color
1
1.5
2
Apparent Magnitude
Apparent Magnitude
0
5
10
15
-0.5
0
0.5
B-V Color
1
1.5
2
Main Sequence Turnoffs of Star Clusters
Here we see a
series of HR
diagrams for
sequentially
older star
clusters that
have been
superimposed
Burbidge and Sandage 1958, Astrophysical Journal
Ages of Star Clusters
Cluster
NGC 752
M 67
Hyades
Pleiades
M 34
Jewelbox
Turnoff Color
0.35
Age
1.1 billion years
0.45
2.5
billion years
Thinking
Question:
Why
with a
0.15 has a
800cluster
million years
turnoff
color
of B-V=1.0
-0.15
100 million
years
never
been
discovered?
-0.10
180 million years
-0.25
16 million years
For Wednesday
Chapter 19 – Milky Way
EP3 Finished on Wednesday