Chapter 15
• Measuring star properties - Section 15.1
– Range of values for stars
– Typical values for stars
• Sections 15.2 and 15.3 =
HR diagram and what astronomers learned from
studying it.
• Minor revisions: HR diagram is really on page 531
but in notes was listed as page 514 (slide 29).
Questions or answers changed on slides 33 and
50 compared to the student notes.
Goals & Outcomes
• Learn some simple astronomical terminology.
• Develop a sense of what scientists know about the
overall universe, its constituents, and our location
• Understand the data that led to the development of
modern cosmology and the Big Bang theory
• Explain how electromagnetic radiation and
astronomical instruments are used to reveal the
properties of stars and galaxies.
• Calculate the distance to a star.
• Describe major star characteristics
Write a list of properties astronomers
might want to measure or know
• Luminosity
– Brightness
– Distance
•
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Surface Temperature
Radius (surface area)
Mass
Composition
Age (we’ll discuss this one at the end of this
PowerPoint)
How Astronomers Measure Luminosity
• Can be measured two ways:
– Measure Surface Temp and Radius
– We’ll discuss both of these again later
– Measure Brightness & calculate distance
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Brightness = How it looks from Earth
Easy or hard?
Measure/calculate distance. Easy or hard?
Use inverse square law to calculate luminosity
See pages 519-520
Luminosity - continued
• HUGE range:
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10-4 Lsun  106 Lsun.
Sun’s Luminosity = 1 Lsun.
Typical 0.1 Lsun  0.01 Lsun. Extremes are less likely.
Extremely luminous stars are MUCH easier to find.
• Nearby, we find MANY more faint stars than
luminous stars
– Why? What does this mean?
– (see page 523, 2nd bullet)
How Astronomers Measure Distances
• Three techniques:
– Parallax (pages 521-523)
– Standard Ruler (not in textbook because not used
for stars… Only for galaxies. We’ll see why.)
– Standard Candle (pages 519-520)
Measuring Distance using Parallax
• Example: Finger on outstretched arm
– Explain on chalkboard.
• Regular figure 15.3, page 521
• Interactive Figure 15.3
• See Lecture Tutorial, pages 35-42 to make your own
parallax calculations
• Shift is called parallax angle or parallactic shift. Depends on:
– Distance to object.
• Angle gets ________ as distance increases. [clicker ? next]
– Separation between eyes/observations (baseline)
• Angle gets ________ as separation increases. [2nd clicker ?]
Two identical objects, one closer, one
further. Which one shows a larger
parallactic shift?
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Further one
Both are the same
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Which produces a larger parallactic shift?
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Parallax - continued
• Can measure the distance to the nearest 2
million stars this way, only ~500 light-years away.
• For those of you who like math:
– Parallax Angle in angle units of arcseconds =
(1 ÷ distance). Distance in units of “parsecs”
– 1 parsec = 206265 AU = 3.26 light years
• This number comes from the fact that there are 206265
arcseconds in one radian.
– Alpha Centauri is 1.3 parsecs away.
• Every other star is further.
Measuring Distances using Standard Ruler
-- Technique not in your textbook -• How big something looks, called “angular size” or “apparent size” and it depends
on:
– Its true size (how many miles/light-years across)
– Distance to Earth
– Math: Angular size = true size / distance.
– Know any 2 of these 3 numbers, calculate the 3rd.
• What we see for stars:
– Sun’s radius is only 700,000 km (440,000 miles, 110 Earths)
– Closest stars are tens of light years away
– Apparent size = 0.005 arcseconds, 40 times smaller than the best visible
telescopes can see. Only see “dot”
– Only can measure angular size using radio interferometry
• Galaxies are big enough to see as more than dots.
– Can use this technique.
Measuring Distances using Standard Candle
• How bright something looks, called “brightness” depends
on:
– Luminosity
• Objects with a known luminosity are called “standard candles”
• Examples include: Type 1a white dwarf supernovas (page 590), Cepheid
variable stars (pages 535, 640-644), and entire galaxies.
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Distance to Earth
Math: Brightness = Luminosity / [4 p (distance)2] See p. 520
Know any 2 of these 3 numbers, calculate the 3rd.
See also pages 640-644
• Often use this relationship to calculate Luminosity after
finding the distance another way.
Measuring Surface Temperature
• Measured several ways
– Color of star (pages 524-525)
• Often take pictures using 2+ filters to measure color
• Space dust can change star color. Dust absorbs blue
light more than red light.
– Wien’s law: peak emission (chapter 5)
– Best method: Identify absorption lines
Measuring Surface Temperature using
Absorption Lines
• Relationship is not obvious, struggled.
– Originally, strong lines = “A” class star,
few/weak lines = “O” class star.
– System didn’t work.
– Women “computers” figured it out.
• Annie Jump Cannon, Williamina Fleming, Antonio Maury, and finally Cecilia
Payne-Gaposchkin.
• Cannon removed duplicate classes, re-ordered from hot to cold.
• She was awesome. Read about her in your textbook!
• Hottest OBAFGKM coldest
• Oh Be A Fine Girl/Guy Kiss Me
– Two new classes of cold brown dwarfs (almost stars): LT (see
footnote on page 525)
• Temperature range: O stars = 30,000-40,000 K. M stars = 3000K
• Sun is a G star. Typical stars are 4000K.
Measuring star masses
• Can measure directly if observe object orbiting the star
– Kepler’s 3rd law. See page 527.
– Measure 2 of these 3: orbit time, speed, orbit dist
• How measure time? Speed? [Distance is tough.]
• What would be easiest to see orbiting a star?
– Another star!
– Binary star. See pages 527-529 for details.
– Half of stars in sky are binary systems
• Interactive Figure 15.8, Additional: Spec. Bin. Movie
• Mass range: 0.08 Msun  100 Msun. Typical 0.2-0.3 Msun
– Less than 0.08 Msun and object stops shrinking before core
heats up to turn on fusion. “Brown Dwarf” or “failed star”
– Bigger than 100 Msun and own solar wind evaporates star.
Measuring Star Composition
• How do we measure composition?
• Range: very little. Stars are all nearly identical.
1. Hydrogen 70-75%
2. Helium 23-28%
3. Other stuff 0-2%
•
Very first stars were made of…
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Next generation have …
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“Population III”
“Population II”
Current generation have …
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“Population I”
Summarizing Star Properties
• Luminosity range: 10-4 Lsun  106 Lsun.
– Sun’s Luminosity = 1 Lsun.
– Typical 0.1 Lsun  0.01 Lsun. Extremes are less likely.
• Distances: 4 LY  100,000 LY (if in Milky Way)
• Temperature range: O stars = 30,000-40,000 K.
M stars = 3000K
– Sun is a G star. Typical stars are 4000K.
• Mass range: 0.08 Msun  100 Msun.
– Typical 0.2-0.3 Msun
• Sun’s Composition: 70% H, 28% He, 2% other
– Very little variation now. Some variation long ago.
Finding patterns
• Now we know how star properties are measured.
• We also know what we find, generally.
• Astronomers wanted to see if any other patterns
exist. Questions like:
– Are the hot stars always big? Small?
– How does mass relate to luminosity? Temperature?
Composition?
– Questions you have were probably the same as those
being asked 100 years ago.
– Hertzsprung and Russell first gathered data on
questions like these.
– They graphed the results in what’s now called the “HR
diagram.” You’ve already seen one in a Lecture Tutorial
exercise on page 55.
HR diagram
• HR = Hertzsprung-Russell
• Horizontal axis: temperature, listed in order of the spectral
classes
– OBAFGKM
– Add 2 new brown dwarf categories, LT
– What’s a brown dwarf?
• Vertical axis: luminosity. Measured relative to The Sun
– Symbol for The Sun = ‫סּ‬
• Sun’s location on HR diag: G class; Luminosity = 1 L‫סּ‬
• Draw a graph in your notes. With your neighbors, decide:
– Where are the hot stars? Cold stars
– Luminous? Faint
– Label the four corners with surface temperature & luminosity
(hot, cold, luminous, dim)
– Clicker questions coming.
Stars in the top right of the HR diagram
are:
1. Luminous
2. Faint
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Stars in the top right of the HR diagram
are:
1. Hot
2. Cold
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Another property shown on
HR diagrams
• On your HR diagram, figure out where the biggest
and smallest radius stars are.
– Clicker questions coming.
Which corner of the HR diagram has the
largest radius stars?
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Top right
Bottom left
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Which star is more luminous on my
drawing?
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Directly On right side
Same
Not enough information
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Which corner of the HR diagram has the
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Top right
Bottom left
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Which color are M stars?
1. Red
2. Blue
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Which color are O stars?
1. Red
2. Blue
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Which star is larger? Both have the same
luminosity.
1. Hotter star
2. Colder star
3. Both the hot & cold star are same size
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What we see in nature
• You now understand what an HR diagram shows.
• Take a look at figure 15.10 on page 531.
• 3 regions on the HR diagram where stars are:
– Main Sequence
– Red Giants (and supergiants)
– White dwarfs
• What can we conclude from this observation:
90% of stars are on the Main Sequence?
• We also now know:
– Red giant stars are dying
– White dwarf stars are dead (although they can be active)
• Novas, white dwarf supernovas.
• When we measure masses we get … but first …
– How do we measure masses?
– Masses on Main Sequence…
NAMES: “Dwarf” stars
• Lots of “dwarfs” in astronomy
– All main sequence stars are called dwarfs
• TERRIBLE name. They’re not all small.
– White dwarfs
• What color are they?
– Black dwarfs
• Cooled off white dwarfs
• NOT on your exams
• DO NOT MIX UP WITH BLACK HOLES.
– Brown dwarfs
• What are these?
• Brown dwarfs ARE fair game for your test.
Which kind of dwarf is biggest?
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Black
Blue
Red
White
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Which type of Main Sequence stars are
easier to make (i.e. more numerous)?
1. Super-bright
2. Super-faint
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Based on figure 15.10, 15.11, which stars are
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2. Colder than Sun
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Ages of stars
• Why do stars die?
• To help us determine what causes this, let’s
use an analogy.
• A car “dies” when it runs out of gasoline.
• What two primary factors determine when it
runs out of gas?
– How quickly it uses gas
– How big the gas tank is.
• Star equivalents to these?
Which kind of main sequence star has a larger
“gas tank”? [How much more?]
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Low mass
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Which kind of main sequence star uses its fuel
faster? [How much more?]
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Based on those two questions, which kind of
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Are there many dead red dwarfs?
1. Yes
2. No
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Properties of stars
• High mass main sequence
• Low mass main sequence
Would you rather live life as a high-mass
star or low mass star?
1. High mass
2. Low mass
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Do stars lose a lot of their mass during their
main sequence lifetimes?
1. Yes
2. No
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Do stars evolve ALONG the main
sequence?
1. Yes
2. No
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REVIEW: The number of gas particles in the core
of stars is ______?
1. increasing
2. decreasing
3. Staying the same
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Which is more luminous?
1. M dwarf
2. M giant
3. Same
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Which is hotter on the outside?
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2. M giant
3. Same
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Stellar evolution
• If we wanted to see how humans changed over
their lives, how could we do that?
• Studying stellar evolution requires either:
– seeing stars change. Can we? Why or why not?
– What else?
• Seeing many stars at different stages of their lives
• How can we do that?
Star Clusters
• To see stars in different stages, look at clusters.
• Clusters = many stars born from same cloud
– All stars at the same distance
• brightness difference = luminosity difference.
– All stars made of the exact same composition
– All stars born at the same time (same age)
• Differences in appearance would be because ….
Which cluster is older?
Decide now. Next slide = clicker.
Cluster A – pay attention to the blue
Cluster B – the Main Sequence
triangles. The band is the Main Sequence
band isn’t shown on this figure.
Which cluster is older?
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Cluster A
Cluster B
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Insufficient information
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The cluster I’ve drawn on the board is: (NOTE:
the answers are different from your notes)
1. Older than 10 billion years
2. Younger than 10 billion years
3. 10 billion yrs
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REVIEW QUESTIONS: Which has a hotter
surface?
1. M dwarf
2. M giant
3. Same
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40
41
42
43
44
45
46
47
48
49
50
Which has the hottest surface?
1.
2.
3.
4.
0
0
0
0
B class main sequence
Red giant
Sun
M dwarf
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
Which is smallest?
1.
2.
3.
4.
0
0
0
0
B class main sequence
Red giant
Sun
M dwarf
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
Which has the coldest surface?
1.
2.
3.
4.
0
0
0
0
B class main sequence
Red giant
Sun
M dwarf
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
Which is NOT fusing Hydrogen into Helium
in the core?
1.
2.
3.
4.
0
0
0
0
B class main sequence
Red giant
Sun
M dwarf
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
Which will have the longest life?
1. B class main sequence
2. Sun
3. M dwarf
0
0
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
Which cluster is oldest?
1. Main sequence turnoff at high lumin.
2. Main sequence turnoff at low lumin.
3. Insufficient information to answer
0
0
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50