My God, it s full of stars! - Stony Brook University · The magnitude scale was quantified in the...
Transcript of My God, it s full of stars! - Stony Brook University · The magnitude scale was quantified in the...
![Page 1: My God, it s full of stars! - Stony Brook University · The magnitude scale was quantified in the 17th and 18th centuries • 1 magnitude difference = ratio of 2.512 in brightness](https://reader033.fdocuments.in/reader033/viewer/2022060604/6059d092a44e2446847d01a9/html5/thumbnails/1.jpg)
“…My God, it’s full of stars!”
AST 248
![Page 2: My God, it s full of stars! - Stony Brook University · The magnitude scale was quantified in the 17th and 18th centuries • 1 magnitude difference = ratio of 2.512 in brightness](https://reader033.fdocuments.in/reader033/viewer/2022060604/6059d092a44e2446847d01a9/html5/thumbnails/2.jpg)
N*
The number of stars in the Galaxy N = N* fs fp nh fl fi fc L/T
![Page 3: My God, it s full of stars! - Stony Brook University · The magnitude scale was quantified in the 17th and 18th centuries • 1 magnitude difference = ratio of 2.512 in brightness](https://reader033.fdocuments.in/reader033/viewer/2022060604/6059d092a44e2446847d01a9/html5/thumbnails/3.jpg)
The Galaxy
M31, the Andromeda Galaxy 2 million light years from Earth
![Page 4: My God, it s full of stars! - Stony Brook University · The magnitude scale was quantified in the 17th and 18th centuries • 1 magnitude difference = ratio of 2.512 in brightness](https://reader033.fdocuments.in/reader033/viewer/2022060604/6059d092a44e2446847d01a9/html5/thumbnails/4.jpg)
The Shape of the Galaxy
A flattened spiral with a central bulge
![Page 5: My God, it s full of stars! - Stony Brook University · The magnitude scale was quantified in the 17th and 18th centuries • 1 magnitude difference = ratio of 2.512 in brightness](https://reader033.fdocuments.in/reader033/viewer/2022060604/6059d092a44e2446847d01a9/html5/thumbnails/5.jpg)
The REAL Shape of the Galaxy
Chen X., et al. 2019 Nature Astronomy
![Page 6: My God, it s full of stars! - Stony Brook University · The magnitude scale was quantified in the 17th and 18th centuries • 1 magnitude difference = ratio of 2.512 in brightness](https://reader033.fdocuments.in/reader033/viewer/2022060604/6059d092a44e2446847d01a9/html5/thumbnails/6.jpg)
The Size of the Galaxy
• Diameter: 100,000 light years • Thickness: 1000 light years • Average distance between stars: 4 ly How do you estimate N*?
![Page 7: My God, it s full of stars! - Stony Brook University · The magnitude scale was quantified in the 17th and 18th centuries • 1 magnitude difference = ratio of 2.512 in brightness](https://reader033.fdocuments.in/reader033/viewer/2022060604/6059d092a44e2446847d01a9/html5/thumbnails/7.jpg)
The Size of the Galaxy
• Diameter: 100,000 light years • Thickness: 1000 light years • Average distance between stars: 4 ly How you estimate N*:
– V=πr2h ly3 - a cylinder – ρ=1/(43 ly3) - star density
– N*=Vρ ~ 1.2 x 1011
![Page 8: My God, it s full of stars! - Stony Brook University · The magnitude scale was quantified in the 17th and 18th centuries • 1 magnitude difference = ratio of 2.512 in brightness](https://reader033.fdocuments.in/reader033/viewer/2022060604/6059d092a44e2446847d01a9/html5/thumbnails/8.jpg)
N* = 4x1011
![Page 9: My God, it s full of stars! - Stony Brook University · The magnitude scale was quantified in the 17th and 18th centuries • 1 magnitude difference = ratio of 2.512 in brightness](https://reader033.fdocuments.in/reader033/viewer/2022060604/6059d092a44e2446847d01a9/html5/thumbnails/9.jpg)
What is a star?
• Objects with sufficient core temperatures and densities to drive stable Hydrogen fusion
• Mass range about 0.076 M¤ to 120 M¤
![Page 10: My God, it s full of stars! - Stony Brook University · The magnitude scale was quantified in the 17th and 18th centuries • 1 magnitude difference = ratio of 2.512 in brightness](https://reader033.fdocuments.in/reader033/viewer/2022060604/6059d092a44e2446847d01a9/html5/thumbnails/10.jpg)
What is not a star?
Objects less massive than about 0.076 M¤
• 0.013 < M/M¤ < 0.076: Brown Dwarf – Fuses Deuterium to 3He, then stops
• <0.013 M¤ : planet – 0.013 M¤ is about 13 Jovian masses – Tcore < 106 K
• Never supports core fusion
– Lower limit set by sphericity
![Page 11: My God, it s full of stars! - Stony Brook University · The magnitude scale was quantified in the 17th and 18th centuries • 1 magnitude difference = ratio of 2.512 in brightness](https://reader033.fdocuments.in/reader033/viewer/2022060604/6059d092a44e2446847d01a9/html5/thumbnails/11.jpg)
What is not a star?
Objects more massive than about 120-150 M¤
• Mass limited by radiation pressure • If not, could collapse to black hole
![Page 12: My God, it s full of stars! - Stony Brook University · The magnitude scale was quantified in the 17th and 18th centuries • 1 magnitude difference = ratio of 2.512 in brightness](https://reader033.fdocuments.in/reader033/viewer/2022060604/6059d092a44e2446847d01a9/html5/thumbnails/12.jpg)
Stars
Observables: • Brightness • Color • Spectra • Position
• Position
![Page 13: My God, it s full of stars! - Stony Brook University · The magnitude scale was quantified in the 17th and 18th centuries • 1 magnitude difference = ratio of 2.512 in brightness](https://reader033.fdocuments.in/reader033/viewer/2022060604/6059d092a44e2446847d01a9/html5/thumbnails/13.jpg)
Brightness • Magnitude scale: rooted in history
• The brightest stars were designated Stars of the first magnitude
• Lesser luminaries were Stars of the second magnitude
• The Greeks recognized 5 magnitudes.
• The eye has an approximately logarithmic response: magnitude differences correspond to brightness ratios
![Page 14: My God, it s full of stars! - Stony Brook University · The magnitude scale was quantified in the 17th and 18th centuries • 1 magnitude difference = ratio of 2.512 in brightness](https://reader033.fdocuments.in/reader033/viewer/2022060604/6059d092a44e2446847d01a9/html5/thumbnails/14.jpg)
Magnitudes Consequently, magnitudes are • Logarithmic, and • Backwards
The brightest stars have the smallest magnitudes
![Page 15: My God, it s full of stars! - Stony Brook University · The magnitude scale was quantified in the 17th and 18th centuries • 1 magnitude difference = ratio of 2.512 in brightness](https://reader033.fdocuments.in/reader033/viewer/2022060604/6059d092a44e2446847d01a9/html5/thumbnails/15.jpg)
Magnitudes The magnitude scale was quantified in the 17th and 18th
centuries • 1 magnitude difference = ratio of 2.512 in brightness • 5 magnitudes = factor of 100 in brightness (2.5125 = 100) • Vega has a magnitude of 0.0 (definition) • Brighter stars have negative magnitudes
– Sirius: mag = -1.6 – Sun: mag = -26 – Full moon: mag = -12 – Venus (at brightest): mag = -4 – Faintest nakest eye stars: mag = +6.5 – Faintest objects detected: mag ~ +30
![Page 16: My God, it s full of stars! - Stony Brook University · The magnitude scale was quantified in the 17th and 18th centuries • 1 magnitude difference = ratio of 2.512 in brightness](https://reader033.fdocuments.in/reader033/viewer/2022060604/6059d092a44e2446847d01a9/html5/thumbnails/16.jpg)
Magnitudes Apparent magnitudes (m): what we see Absolute magnitudes (M): pegged to luminosity M is the magnitude you would see at a distance of
10 parsecs (32.6 light years) M¤ = 4.8 Distance modulus (DM): a measure of distance
DM = m - M = 5 log(d) -5
![Page 17: My God, it s full of stars! - Stony Brook University · The magnitude scale was quantified in the 17th and 18th centuries • 1 magnitude difference = ratio of 2.512 in brightness](https://reader033.fdocuments.in/reader033/viewer/2022060604/6059d092a44e2446847d01a9/html5/thumbnails/17.jpg)
Colors • The eye perceives colors
only in the brightest stars
• Stars have colors because, to a first approximation, they radiate as blackbodies
![Page 18: My God, it s full of stars! - Stony Brook University · The magnitude scale was quantified in the 17th and 18th centuries • 1 magnitude difference = ratio of 2.512 in brightness](https://reader033.fdocuments.in/reader033/viewer/2022060604/6059d092a44e2446847d01a9/html5/thumbnails/18.jpg)
Astronomical Colors Colors are differences in magnitudes (= ratio of fluxes) • B-V color = B mag - V mag • Larger values are redder colors • Smaller values are bluer colors • By definition, the colors of Vega = 0
Color is indicative of temperature Red indicates cool Blue indicates hot
![Page 19: My God, it s full of stars! - Stony Brook University · The magnitude scale was quantified in the 17th and 18th centuries • 1 magnitude difference = ratio of 2.512 in brightness](https://reader033.fdocuments.in/reader033/viewer/2022060604/6059d092a44e2446847d01a9/html5/thumbnails/19.jpg)
Bandpasses
Human response to visible light (cone cells)
![Page 20: My God, it s full of stars! - Stony Brook University · The magnitude scale was quantified in the 17th and 18th centuries • 1 magnitude difference = ratio of 2.512 in brightness](https://reader033.fdocuments.in/reader033/viewer/2022060604/6059d092a44e2446847d01a9/html5/thumbnails/20.jpg)
Astronomical Bandpasses
UBVRI filter responses
![Page 21: My God, it s full of stars! - Stony Brook University · The magnitude scale was quantified in the 17th and 18th centuries • 1 magnitude difference = ratio of 2.512 in brightness](https://reader033.fdocuments.in/reader033/viewer/2022060604/6059d092a44e2446847d01a9/html5/thumbnails/21.jpg)
Spectra of Stars • All stars have an opaque photosphere
– continuum emission
• Most stars have absorption line spectra – cool photosphere
• Some also show emission lines – hot or extended circumstellar gas
![Page 22: My God, it s full of stars! - Stony Brook University · The magnitude scale was quantified in the 17th and 18th centuries • 1 magnitude difference = ratio of 2.512 in brightness](https://reader033.fdocuments.in/reader033/viewer/2022060604/6059d092a44e2446847d01a9/html5/thumbnails/22.jpg)
Spectral Types
• O • B • A • F • G • K • M • L • T • Y
![Page 23: My God, it s full of stars! - Stony Brook University · The magnitude scale was quantified in the 17th and 18th centuries • 1 magnitude difference = ratio of 2.512 in brightness](https://reader033.fdocuments.in/reader033/viewer/2022060604/6059d092a44e2446847d01a9/html5/thumbnails/23.jpg)
Spectral Types An empirical scale based on appearance of the spectrum • Originally based on the strengths of absorption lines:
A, B, C, D, E …
• Now reordered into a temperature sequence
• Spectral types have subtypes, e.g., B0, B1, B2 … B9 • Spectral types have qualifiers, e.g., M5e
![Page 24: My God, it s full of stars! - Stony Brook University · The magnitude scale was quantified in the 17th and 18th centuries • 1 magnitude difference = ratio of 2.512 in brightness](https://reader033.fdocuments.in/reader033/viewer/2022060604/6059d092a44e2446847d01a9/html5/thumbnails/24.jpg)
Spectral Types • O: Hottest stars; temps from ~20,000K to > 100,000K. Weak
helium absorption. • B: Temperatures from 10,000 to 20,000K. Noticeably blue.
Examples: Rigel, in Orion, and Spica, in Virgo. • A: Temperatures from 8000-10,000K. They appear white. Strong
absorption lines of hydrogen. Examples: Vega, Altair, Sirius. • F: slightly hotter than the Sun. Absorption lines of metals appear.
Procyon is an F star. • G: temperatures between 5000 and 6000K. Appear yellow.
Examples: Sun, α Centauri, Capella. • K: Temperatures 3000 - 5000K. Appear orange. Arcturus is a K
star. • M: the coolest stars; 2000 - 3000K. Molecules can survive (H2O,
CO, VO, TiO). Noticeably red. Examples: Betelgeuse, Antares. • L, T, and Y objects are brown dwarfs, not stars
![Page 25: My God, it s full of stars! - Stony Brook University · The magnitude scale was quantified in the 17th and 18th centuries • 1 magnitude difference = ratio of 2.512 in brightness](https://reader033.fdocuments.in/reader033/viewer/2022060604/6059d092a44e2446847d01a9/html5/thumbnails/25.jpg)
Spectral Type Mnemonics
OBAFGKM • O Be A Fine Girl/Guy, Kiss Me. • Old Bald And Fat Generals Keep Mistresses. • O Boy, An F Grade Kills Me. • Only Boring Astronomers Find Gratification Knowing
Mnemonics
![Page 26: My God, it s full of stars! - Stony Brook University · The magnitude scale was quantified in the 17th and 18th centuries • 1 magnitude difference = ratio of 2.512 in brightness](https://reader033.fdocuments.in/reader033/viewer/2022060604/6059d092a44e2446847d01a9/html5/thumbnails/26.jpg)
The Hertzsprung-
Russell Diagram
• X-axis: color, temp, spectral type
• Y axis: absolute magnitude, luminosity
Reveals giants, dwarfs, and the main sequence (distances from parallax)
![Page 27: My God, it s full of stars! - Stony Brook University · The magnitude scale was quantified in the 17th and 18th centuries • 1 magnitude difference = ratio of 2.512 in brightness](https://reader033.fdocuments.in/reader033/viewer/2022060604/6059d092a44e2446847d01a9/html5/thumbnails/27.jpg)
Luminosity Classes • I: supergiants • II: bright giants • III: giants • IV: subgiants • V: dwarfs - the main sequence • VI: subdwarfs • WD: white dwarfs
A luminosity, or pressure sequence. Observable in the spectra.
The Sun is G2V
![Page 28: My God, it s full of stars! - Stony Brook University · The magnitude scale was quantified in the 17th and 18th centuries • 1 magnitude difference = ratio of 2.512 in brightness](https://reader033.fdocuments.in/reader033/viewer/2022060604/6059d092a44e2446847d01a9/html5/thumbnails/28.jpg)
H-R Diagram
![Page 29: My God, it s full of stars! - Stony Brook University · The magnitude scale was quantified in the 17th and 18th centuries • 1 magnitude difference = ratio of 2.512 in brightness](https://reader033.fdocuments.in/reader033/viewer/2022060604/6059d092a44e2446847d01a9/html5/thumbnails/29.jpg)
Understanding the H-R Diagram
• Most stars are on the main sequence. – Stars spend most of their life on the main
sequence – Most stars are faint and red
![Page 30: My God, it s full of stars! - Stony Brook University · The magnitude scale was quantified in the 17th and 18th centuries • 1 magnitude difference = ratio of 2.512 in brightness](https://reader033.fdocuments.in/reader033/viewer/2022060604/6059d092a44e2446847d01a9/html5/thumbnails/30.jpg)
![Page 31: My God, it s full of stars! - Stony Brook University · The magnitude scale was quantified in the 17th and 18th centuries • 1 magnitude difference = ratio of 2.512 in brightness](https://reader033.fdocuments.in/reader033/viewer/2022060604/6059d092a44e2446847d01a9/html5/thumbnails/31.jpg)
Understanding the H-R Diagram
• Most stars are on the main sequence. – Stars spend most of their life on the main
sequence – Most stars are faint and red
• Giants and supergiants are visible from great distances because they are very luminous. – Giants and supergiants are rare.
![Page 32: My God, it s full of stars! - Stony Brook University · The magnitude scale was quantified in the 17th and 18th centuries • 1 magnitude difference = ratio of 2.512 in brightness](https://reader033.fdocuments.in/reader033/viewer/2022060604/6059d092a44e2446847d01a9/html5/thumbnails/32.jpg)
![Page 33: My God, it s full of stars! - Stony Brook University · The magnitude scale was quantified in the 17th and 18th centuries • 1 magnitude difference = ratio of 2.512 in brightness](https://reader033.fdocuments.in/reader033/viewer/2022060604/6059d092a44e2446847d01a9/html5/thumbnails/33.jpg)
![Page 34: My God, it s full of stars! - Stony Brook University · The magnitude scale was quantified in the 17th and 18th centuries • 1 magnitude difference = ratio of 2.512 in brightness](https://reader033.fdocuments.in/reader033/viewer/2022060604/6059d092a44e2446847d01a9/html5/thumbnails/34.jpg)
Binary Stars
Stellar masses can be determined using Newton’s version of Kepler’s third law,
(M1+M2)= (4π2/G) a3/P2
![Page 35: My God, it s full of stars! - Stony Brook University · The magnitude scale was quantified in the 17th and 18th centuries • 1 magnitude difference = ratio of 2.512 in brightness](https://reader033.fdocuments.in/reader033/viewer/2022060604/6059d092a44e2446847d01a9/html5/thumbnails/35.jpg)
Types of Binary Stars
• Visual pairs • Eclipsing binaries ⇒
• ⇐ Spectroscopic binaries
![Page 36: My God, it s full of stars! - Stony Brook University · The magnitude scale was quantified in the 17th and 18th centuries • 1 magnitude difference = ratio of 2.512 in brightness](https://reader033.fdocuments.in/reader033/viewer/2022060604/6059d092a44e2446847d01a9/html5/thumbnails/36.jpg)
Spectroscopic Binary
![Page 37: My God, it s full of stars! - Stony Brook University · The magnitude scale was quantified in the 17th and 18th centuries • 1 magnitude difference = ratio of 2.512 in brightness](https://reader033.fdocuments.in/reader033/viewer/2022060604/6059d092a44e2446847d01a9/html5/thumbnails/37.jpg)
Star Clusters All members have the same • Age • Distance from Earth • Composition
A testing ground for theories of • Star formation • Stellar evolution
![Page 38: My God, it s full of stars! - Stony Brook University · The magnitude scale was quantified in the 17th and 18th centuries • 1 magnitude difference = ratio of 2.512 in brightness](https://reader033.fdocuments.in/reader033/viewer/2022060604/6059d092a44e2446847d01a9/html5/thumbnails/38.jpg)
Globular Clusters
47 Tucanae
![Page 39: My God, it s full of stars! - Stony Brook University · The magnitude scale was quantified in the 17th and 18th centuries • 1 magnitude difference = ratio of 2.512 in brightness](https://reader033.fdocuments.in/reader033/viewer/2022060604/6059d092a44e2446847d01a9/html5/thumbnails/39.jpg)
Globular Clusters
• Old and dense • Good place to look for advanced
civilizations?
![Page 40: My God, it s full of stars! - Stony Brook University · The magnitude scale was quantified in the 17th and 18th centuries • 1 magnitude difference = ratio of 2.512 in brightness](https://reader033.fdocuments.in/reader033/viewer/2022060604/6059d092a44e2446847d01a9/html5/thumbnails/40.jpg)
Galactic, or Open Clusters
h and χ Persei
![Page 41: My God, it s full of stars! - Stony Brook University · The magnitude scale was quantified in the 17th and 18th centuries • 1 magnitude difference = ratio of 2.512 in brightness](https://reader033.fdocuments.in/reader033/viewer/2022060604/6059d092a44e2446847d01a9/html5/thumbnails/41.jpg)
Pleiades
![Page 42: My God, it s full of stars! - Stony Brook University · The magnitude scale was quantified in the 17th and 18th centuries • 1 magnitude difference = ratio of 2.512 in brightness](https://reader033.fdocuments.in/reader033/viewer/2022060604/6059d092a44e2446847d01a9/html5/thumbnails/42.jpg)
Open Clusters
• Young and sparse • Probably not a good place to look for
advanced civilizations
![Page 43: My God, it s full of stars! - Stony Brook University · The magnitude scale was quantified in the 17th and 18th centuries • 1 magnitude difference = ratio of 2.512 in brightness](https://reader033.fdocuments.in/reader033/viewer/2022060604/6059d092a44e2446847d01a9/html5/thumbnails/43.jpg)
Stars • Stars are in hydrostatic equilibrium: gravitational
pressure balances gas pressure.
• Stars are generally stable for long times
• Stellar properties are determined solely by their mass, composition, age, and rotation rate.
• The properties of main sequence stars are largely determined by their masses.
• Single stars are spherical, unless distorted by rotation
![Page 44: My God, it s full of stars! - Stony Brook University · The magnitude scale was quantified in the 17th and 18th centuries • 1 magnitude difference = ratio of 2.512 in brightness](https://reader033.fdocuments.in/reader033/viewer/2022060604/6059d092a44e2446847d01a9/html5/thumbnails/44.jpg)
Relevance to AST 248
• N* ~ 4x1011
• Planets orbit stars • Planetary temperature is set by (L*/d2)1/4
– More luminous stars have wider habitable zones
• Stellar lifetime is set by mass and luminosity – More luminous stars have shorter lifetimes