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Astronomy 101 Chapter 15
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Outlines • Chapter 15 “Surveying the Stars”
– What kinds of star out there? – How can we measure their properties?
• Chapter 16 “Star Birth” – How do they form?
• Chapter 17 “Star Stuff” – How do stars live/evolve?
• Chapter 18 “The Stellar Graveyard” – How do stars die?
Nova -- Novae Small explosion on stellar surface causes nova (star suddenly becomes bright).
The explosion is caused by the material falling from a companion star.
Binary star
Two stars orbiting around each other.
Prof. Fred Walter is an expert of Novae.
Supernova – Supernovae Two types of suparnova (Type I and Type II)
Type II Supernova
Type I Supernova Computer simulation (By Prof. Alan Calder at SBU)
Stellar Clusters Open Clusters
Cluster in Formation
M45 (Pleiades)
Open clusters are mostly in the Milky Way.
Stellar Clusters Globular Clusters
Concentrations of stars. All stars in stellar clusters formed almost at the same time (hence, all stars have the same age).
Mostly outside the Milky Way (MW) disk, but still inside the MW.
15.1 Properties of Stars
Our goals for learning: • How do we measure stellar luminosities? • How do we measure stellar temperatures? • How do we measure stellar masses?
In science, we measure stuffs and understand them. But how?
Remember: In case of Sun
• We know the mass and luminosity (amount of lights per second) of the Sun.
• From those numbers, we made the model of the Sun.
• For other stars, we have to measure those numbers first, and then make models (understand it!).
Luminosity:
Amount of power a star radiates
(energy per second = watts) Apparent brightness:
Amount of starlight that reaches Earth
(energy per second per square meter)
Insert TCP 5e Figure 15.1
Thought Question
These two stars have about the same luminosity -- which one appears brighter?
A. Alpha Centauri B. The Sun
Luminosity passing through each sphere is the same Area of sphere: 4π (radius)2 Divide luminosity by area to get brightness
The same amount of energy passes through these three squares. The area of the squares increases with distance à energy per unit area decreases.
The relationship between apparent brightness and luminosity depends on distance: Luminosity Brightness = 4π (distance)2 We can determine a star’s luminosity if we can measure its distance and apparent brightness: Luminosity = 4π (distance)2 x (Brightness)
Thought Question
How would the apparent brightness of Alpha Centauri change if it were three times farther away?
A. It would be only 1/3 as bright B. It would be only 1/6 as bright C. It would be only 1/9 as bright D. It would be three times brighter
Parallax is the apparent shift in position of a nearby object against a background of more distant objects
Parallax angle
Because the Earth orbits around the Sun every year, the direction of a star changes with season.
Angle=1 arcsecond
Because the Earth orbits around the Sun every year, the direction of a star changes with season.
Distance = 1 parsec
Parallax and Distance
p = parallax angle
d (in parsecs) = 1p (in arcseconds)
d (in light-years) = 3.26 × 1p (in arcseconds)
Parallax angle gives us distance.
Now you know • Apparent brightness
• how much energy you receive on Earth • Distance
Measuring Luminosity
You can calculate Luminosity
The Magnitude Scale
€
m = apparent magnitude , M = absolute magnitude
apparent brightness of Star 1apparent brightness of Star 2
= (1001/ 5)m1−m2
luminosity of Star 1luminosity of Star 2
= (1001/ 5)M 1−M 2
A 1st magnitude star is 100 times brighter than 6 magnitude star.
Thermal Radiation
• Nearly all objects emit thermal radiation, including stars, planets, you…
• An object’s thermal radiation spectrum depends
on only one property: its temperature
Properties of Thermal Radiation Hotter objects 1. emit more light at all frequencies per unit area. 2. emit photons with a higher average energy. 3. are bluer (whiter).
Bluer Redder
Color of Stars
Star’s color depends on its surface temperature. More massive stars have higher temperature, therefore are bluer (whiter) than less massive stars.
Solid
Molecules
Neutral Gas
Ionized Gas (Plasma)
So! Spectral lines (level of ionization) also tell us star’s temperature
10 K
102 K
103 K
104 K
105 K
106 K Phase of Matter
1. Phase of matter depends on temperature.
2. Spectral line emission and absorption depends on the phase.
Lines in a star’s spectrum correspond to a spectral type that reveals its temperature (Hottest) O B A F G K M (Coolest)
(Hottest) O B A F G K M (Coolest)
Remembering Spectral Types
• Oh, Be A Fine Girl, Kiss Me
• Only Boys Accepting Feminism Get Kissed Meaningfully
Pioneers of Stellar Classification
• Annie Jump Cannon and her collaborators at Harvard laid the foundation of modern stellar classification
Most luminous stars: 106 LSun Least luminous stars: 10-4 Lsun (LSun is luminosity of Sun)
Most massive stars: ? MSun Least luminous stars: ? Msun (MSun is mass of Sun)
Question
The orbit of a binary star system depends on strength of gravity REMEMBER: Newton’s law of gravity. Gravity (Mass) Orbital Motion
Types of Binary Star Systems
• Visual Binary • Eclipsing Binary • Spectroscopic Binary
About half of all stars are in binary systems
Visual Binary
We can directly observe the orbital motions of these stars
Newton’s law of gravity is universal à Binary stars show elliptical orbits like Earth’s orbit around the Sun
SBU Prof. Mike Simon is doing this.
Spectroscopic Binary
Again, we cannot see the orbit, but can determine the orbit by measuring Doppler shifts
We measure mass using gravity
Direct mass measurements are possible only for stars in binary star systems
p = period
a = average separation
p2 = a3 4π2
G (M1 + M2)
Need 2 out of 3 observables to measure mass:
1) Orbital Period (p) 2) Orbital Separation (a or r = radius) 3) Orbital Velocity (v)
For circular orbits, v = 2πr / p r M
v
Supermassive Black Hole at the Center of Milky Way 4 million times the mass of our Sun
Gravity (Mass)
Orbital Motion
What have we learned? • How do we measure stellar luminosities?
– If we measure a star’s apparent brightness and distance, we can compute its luminosity with the inverse square law for light
– Parallax tells us distances to the nearest stars
• How do we measure stellar temperatures? – A star’s color and spectral type both reflect its
temperature
What have we learned? • How do we measure stellar masses?
– Newton’s version of Kepler’s third law tells us the total mass of a binary system, if we can measure the orbital period (p) and average orbital separation of the system (a)
15.2 Patterns Among Stars
Our goals for learning: • What is a Hertzsprung-Russell diagram? • What is the significance of the main
sequence? • What are giants, supergiants, and white
dwarfs? • Why do the properties of some stars vary?
Stellar Classification • Library Classification (example in our life)
– Alphabetical indexing – Subject access (politics, economics, etc)
• Stellar Classification
– Many types of stars – Need to categorize (classify) them before understanding. – Of course, we want to do this in a scientific way.
Temperature (Color)
Lum
inos
ity
Stellar Classification on an H-R diagram. An H-R diagram plots the luminosity and temperature (color) of stars
Hertzsprung-Russell diagram
Most stars fall somewhere on the main sequence of the H-R diagram Sun is a main sequence star.
Main Sequence
Stars with lower T and higher L than main-sequence stars must have larger radii: giants and supergiants
Large radius
Main Sequence
They are redder àLower Temperature à Fainter (in unit area) à Have to be bigger to be bright (higher L)
Small radius
Stars with higher T and lower L than main-sequence stars must have smaller radii: white dwarfs
Main Sequence
They are bluer (whiter) àHigher Temperature à Brighter (in unit area) à Have to be smaller to be faint (lower L)
Temperature
Lum
inos
ity
H-R diagram depicts:
Temperature
Color
Spectral Type
Luminosity
Radius
Color Blue Red
High Low
Spectral Type O B A F G K M
Small radius
Large radius
Hertzsprung-Russell diagram
• An H-R diagram is a good, scientific way to classify stars.
• The Location of a star in H-R diagram gives you an idea of characteristics of the star.
Main-sequence stars are fusing hydrogen into helium in their cores like the Sun Luminous main-sequence stars are hot (blue) Less luminous ones are cooler (yellow or red)
Luminous and Hotter (blue)
Less luminous and Cooler
Sun is a main-sequence star.
Mass measurements of main-sequence stars show that the hot, blue stars are much more massive than the cool, red ones
Mass of main-sequence star Luminous and Hotter (blue) Massive
Less luminous and Cooler Less massive
The mass of a normal, hydrogen-burning star determines its luminosity and spectral type!
High-mass stars
Low-mass stars Why more massive main-sequence star is more luminous and bluer?
Two forces balance Gravity ~ Pressure
Remember: Gravitational Equilibrium
More mass à Stronger gravity à Higher core pressure à Higher core temperature à More nuclear fusion (more energy) à More luminosity Nuclear Fusion
Stellar Properties Review Luminosity: from brightness and distance
10-4 LSun - 106 LSun Temperature: from color and spectral type
3,000 K - 50,000 K Mass: from period (p) and average separation (a)
of binary-star orbit
0.08 MSun - 100 MSun
(0.08 MSun) (100 MSun)
(100 MSun) (0.08 MSun)
Mass & Lifetime Massive star
Less massive star
Guess Which star dies first? In other word, which star exhausts the fuel for nuclear fusion first?
A = Massive star B = Less massive star
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
Until core hydrogen (10% of total) is used up
Note: the age of the Universe is 14 billion years.
Main-Sequence Star Summary High Mass:
High Luminosity Short-Lived Large Radius Blue
Low Mass:
Low Luminosity Long-Lived Small Radius Red
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 after exhausting their
core hydrogen: giants and supergiants • Most stars end up small and white after fusion has ceased:
white dwarfs
After the lifetime in main-sequence, a star evolves to the giants/supergiants part of an H-R diagram.
Stellar envelope (outer layer) expands à Cool down à Bigger and Redder
Main-sequence à Giants/Supergiants
Planetary Nebula
An expanding outer layer of a star eventually escapes from star’s gravitational field.
Planetary nebula is an expanding envelope (outer layer) of star.
White Dwarf
At the end, The stellar envelope (outer layer) keeps expanding. The core shrink to become white dwarf.
If it’s a massive star, the core becomes even more massive, dense object (i.e., neutron star or black hole).
What have we learned? • What is a Hertzsprung-Russell diagram?
– An H-R diagram plots stellar luminosity of stars versus surface temperature (or color or spectral type)
• What is the significance of the main sequence? – Normal stars that fuse H to He in their cores
fall on the main sequence of an H-R diagram – A star’s mass determines its position along the
main sequence (high-mass: luminous and blue; low-mass: faint and red)
What have we learned? • What are giants, supergiants, and white
dwarfs? – All stars become larger and redder after core
hydrogen burning is exhausted: giants and supergiants
– Most stars end up as tiny white dwarfs after fusion has ceased
• Why do the properties of some stars vary? – Some stars fail to achieve balance between
power generated in the core and power radiated from the surface
15.3 Star Clusters
Our goals for learning: • What are the two types of star clusters? • How do we measure the age of a star
cluster?
Open cluster: A few thousand loosely packed stars All member stars formed almost at the same time, so they have
the same age.
Globular cluster: Up to a million or more stars in a dense ball bound together by gravity All member stars formed almost at the same time, so they have the same age.
Pleiades now has no stars with life expectancy less than around 100 million years
Main-sequence turnoff Massive stars have
short lifetimes on main-sequence, and must have moved to giants/supergiants.
No star Giants Supergiants
Detailed modeling of the oldest globular clusters reveals that they are about 13 billion years old
Born very early in the Universe.
Globular Cluster
What have we learned? • What are the two types of star clusters?
– Open clusters are loosely packed and contain up to a few thousand stars
– Globular clusters are densely packed and contain hundreds of thousands of stars
• How do we measure the age of a star cluster? – A star cluster’s age roughly equals the life
expectancy of its most massive stars still on the main sequence