stars
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Stars. … how I wonder what you are. Goals. Tie together some topics from earlier in the semester to learn about stars: How do we know how far away stars are? How do we know how bright they really are? What are they like? Temperature Radius Mass What categories can we place them in?. - PowerPoint PPT PresentationTRANSCRIPT
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StarsStars
… how I wonder what you are.
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GoalsGoals
Tie together some topics from earlier in the semester to learn about stars:
• How do we know how far away stars are?• How do we know how bright they really
are?• What are they like?
– Temperature– Radius– Mass
• What categories can we place them in?
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Distances
• How do we perceive distances here on Earth?
• How do we know A is closer than B?• Can we apply these to objects in space?• Can we apply these to objects beyond
the solar system?• How do we know how far away the
stars are?
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Stellar Parallax
• Recall from Lecture 1B:• One proof of a heliocentric
Universe is stellar parallax.• Copernicus thought stars
must be too far away.• Nearest star: Proxima
CentauriParallax angle = 0.76 arcsec
• Recall:Tycho’s precision = 1 arcmin
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The Parsec• Triangles:
tan = opposite/adjacent
• For small angles: tan = = (1 AU)/Distance
Distance = (1 AU)/• What is the distance of an
object with = 1 arcsec?Distance = 206,265
AU• Call this distance 1 parsec
(pc)• 1 pc = 206,265 AU = 3.3
lightyears
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Distances
• 1 parsec = distance with a parallax of 1 arcsecond.
• 1 lightyear = distance light travels in one year.
• 1 pc = 206,265 AU = 3.3 lightyears• Closest star: Proxima Centauri
= 0.76 arcsecDistance = 1.3 pc or 4.3 lightyears
arcsec)(in parallax
1 parsecs)(in Distance
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How far is this?
The Sun
New York
Alpha Centauri
Hawaii
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The Solar Neighborhood
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Star light, star bright
• In Lab 1 we talked about stellar magnitudes.
• Vega is magnitude 0 Polaris is magnitude 2.5
• While Vega is brighter than Polaris, Vega is also a lot closer to us.
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Apparent and Absolute
• Apparent Magnitude = the brightness (magnitude) of a star as seen from the Earth. m– Depends on star’s total energy radiated
(Luminosity) and its distance
• Absolute Magnitude = the brightness (magnitude) of a star at a distance of 10 pc. M– Only depends on a star’s luminosity
10pc
distancelog5 10Mm
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example
• Our Sun:– m = -26.8,– distance = 1/206,265 = 4.8 x 10-6 pcSo: M = 4.8
• Polaris:– m = 2.5,– distance = 132 pcSo: M = -3.1
• Polaris is 1500 times more luminous than the Sun!
10pc
distancelog5 10Mm
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Ste
llar
Sp
ectr
a
Stellar Temperatures
How hot are stars?
• In Lecture 2A we learned about blackbody spectra and temperature.
• Since different stars have different colors, different stars must be different temperatures.
Hot
Cool
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Spectral Classificati
ons
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Temperature and Spectral Type
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Binary Stars• Most stars in
the sky are in multiple systems.
• Binaries, triplets, quadruplets, etc….– Alberio– Alcor and Mizar
• The Sun is in the minority by being single.
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Types of Binaries• Visual – You see both
stars• Spectroscopic – You see
one star, but you see the Doppler shift (lecture 2B) due to its orbital motion.– Double-line – see lines
from both stars– Single-line – see only one
set of lines
• Eclipsing – One star passes directly across the other.
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NPOI Observations of Mizar A (1 Ursa Majoris)
Mizar, 88 light years distant, is the middle star in the handle of the Big Dipper. It was the first binary star system to be imaged with a telescope. Spectroscopic observations show periodic
Doppler shifts in the spectra of Mizar A and B, indicating that they are each binary stars. But they were too close to be directly imaged -until 2 May 1996, when the NPOI produced the first
image of Mizar A. That image was the highest angular resolutionimage ever made in optical astronomy. Since then, the NPOI has
observed Mizar A in 23 different positions over half the binary orbit. These images have been combined here to make a movie of the orbit. As a reference point, one component has been fixed at the map center; in reality, the two stars are of comparable size
and revolve about a common central position.
Orbital Phase: 000o
•These categories are purely observer dependent.
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Stellar MassesHow massive are stars?• In Lecture 1B we learned about Kepler’s
Laws.• Kepler’s Third Law relates Period to
Semimajor axis. But also Mass.
• Where M is the Total Mass of the binary.• Most stars have masses calculated this way.
32 aP
32
2 4a
GMP
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Stellar Radii
How big are stars?• We see stars have
different luminosities and different temperatures.
• Stars have different sizes.• If you know:
– Distance– Angular size
• Learn real size.
50 mas
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Stars are small
• Betelgeuse is the only star big enough to directly see its surface with a normal telescope.
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Interferometry
• Combine the light from two or more telescopes to simulate the RESOLUTION of one giant telescope.
VLA - radio
NPOI - optical
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Optical Interferometry
• NPOI simulates a single optical telescope 65 meters in diameter.
• Resolve stars as small as 1.5 mas!PTI - infrared
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Angular versus Linear
Supergiants, Giants and Dwarfs
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H-R Diagram• Can order the stars we see by the
property of temperature and luminosity (or absolute magnitude).
Prominent stars
Nearby Stars
Brightest Stars
1000 pc Stars
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Spectroscopic Parallax• If you know how luminous a star REALLY is
and how bright it looks from Earth, you can determine how far away it must be to look that faint.
• For any star in the sky, we KNOW:– Apparent Magnitude– Spectral Type (O, B, A, F, G, K, M)– Luminosity Class (Main Sequence, Giant, etc…).
These are denoted by a roman numeral (V, III, I,…).
• Use H-R Diagram to figure out how luminous the star really is (M). With (m) one gets distance.
• Works well out to 10,000 pc.
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example
• Deneb is A2Ia star– A2 Blue star– Ia Supergiant– m = 1.25– M = -8.8So: distance = 1000 pc
10pc
distancelog5 10Mm