Date post: | 28-Dec-2015 |
Category: |
Documents |
Upload: | hilda-janis-parsons |
View: | 225 times |
Download: | 8 times |
H205 Cosmic Origins
Properties of Stars (Ch. 15)The Milky Way (Ch. 19)
EP3 Due Wednesday
APOD
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!)
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?
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
2distance 4π
LuminosityBrightnessApparent
How Far Away Are Stars?If we know a star’s apparent AND absolute brightness, we can calculate its distance
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?
brightness changes as 1/distance2
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 observerStellar parallax uses the Earth’s orbit as the baseline
Parallax
1 AUdistance
Angle = p
(p)sin distance
AU 1
parsecs)(in distance arcsecs)in ( p
1
A parsec is the distance at which 1 AU subtends an angle of 1 arc sec
Parsec: the distance to an object with a stellar parallax of one arc second
The parallax of Alpha Centauri = 0.76 arcseconds
A parallax of ~0.001 arc secondsis the smallest we can measure
What is a Parsec???
1 parsec = 3.26 light years
A star at a distance of 1 parsec showsa parallax of 1 arc second How big is one
arc second?
The size of adime at adistance of2.3 miles!
How Big Are Stars?We can’t see the stars’diameters through a telescope.Stars are so far away that wesee 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’stemperature?
Luminosity depends on….
TEMPERATURE -the hotter a star is,the brighter it is.
DIAMETER –the bigger a star is,the brighter it is.
Stellar RadiiWe can’t see the stars’ diameters through a telescope. Stars are so far away that wesee them just as points of light.
If we know a star’s temperature, apparent magnitude, and distance, we can calculate its radius
Temperature from
Stars range in size from about the size of the Earth to hundreds of times the Sun’s diameter
424Luminosity TR
Luminosity from parallax and apparent magnitude
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!)
StarDistance
(LY)Temperature
(K)Absolute
Magnitude
Sun 0.000015 5800 4.8
Sirius 9 9600 1.4
Canopus 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
The Nearest Stars
StarDistance
(LY) TemperatureAbsolute
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
The HRDiagram
Hertzsprung Russell Diagram
-10
-5
0
5
10
15
20
050001000015000200002500030000Temperature (K)
Ab
so
lute
Ma
gn
itu
de
Giants andSupergian
ts
White Dwarf
MainSequence
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
Stars come in many sizes and colors
But only certain sizes
and colors are allowed!
HR Diagram Simulator
Most stars occur in these main groups in the luminosity-
temperature diagram
Main Sequence Giants Supergiants White DwarfsB
RIG
HTN
ESS
TEMPERATURE
The Main Sequence
The sun is an
ordinary, yellow main
sequence starB
RIG
HTN
ESS
TEMPERATURE
Giants and Supergiants are cooler and very largeB
RIG
HTN
ESS
TEMPERATURE
Supergiants
Giants
White dwarfs are small and
hotter
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
0
5
10
15
-0.5 0 0.5 1 1.5 2B-V Color
Ap
par
ent
Mag
nit
ud
e
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
WHYWHYWHY???
Mass measurements of main-sequence stars in binary star systems show that the hot, blue stars are much more massive than the cool, red ones
High-mass stars
Low-mass stars
Main-sequence stars are fusing hydrogen into helium in their cores like the Sun
massive main-sequence stars are hot (blue) and luminous
Less massive stars are cooler (yellow or red)and fainter
The mass of a normal, hydrogen-burning star determines its luminosity and temperature!
High-mass stars
Low-mass stars
Mass & LifetimeSun’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
•104-106 stars•old!•compact balls of stars•high star density
Open Clusters
•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.
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
Stars like the SUN
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
The HR diagrams of clusters of different
ages look very different
Jewelbox
0
5
10
15
-0.5 0 0.5 1 1.5 2B-V Color
Ap
pa
ren
t M
ag
nit
ud
e
M 670
5
10
15
-0.5 0 0.5 1 1.5 2B-V Color
Ap
pa
ren
t M
ag
nit
ud
e
Main Sequence Turnoffs of Star Clusters
Burbidge and Sandage 1958, Astrophysical Journal
Here we see a series of HR diagrams for sequentially older star clusters that have been superimposed
Ages of Star Clusters
Cluster Turnoff Color Age
NGC 752 0.35 1.1 billion years
M 67 0.45 2.5 billion years
Hyades 0.15 800 million years
Pleiades -0.15 100 million years
M 34 -0.10 180 million years
Jewelbox -0.25 16 million years
Thinking Question: Why has a cluster with a turnoff color of B-V=1.0 never been discovered?
For WednesdayChapter 19 – Milky Way
EP3 Finished on Wednesday