Concepts from ASTR1010&

Names, Catalogs, URLs

ASTR 3010

Lecture 2

Chapter 1 & 4

Luminosity, flux density, and surface brightness • Luminosity = Total energy emitted by the source per unit time (ergs/sec)o independent on the distance to the sourceo Lsun = 3.825×1026 W = 3.825×1033 ergs/sec

• Apparent Brightness = luminosity/areao Flux density (aka irradiance)

o Flux density of the Sun at the Earth position1,370 W/m2 = Solar constant

• Surface brightness = flux density at the surface of the source over 1 steradian

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Flux ≡LA=L4πr2

=Et⋅ A

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σ ≡F

solid _ angle=FΩ=sπ

Blackbody radiation• For a given blackbody of temperature T, the surface brightness is

Stefan-Boltzmann Law, and the wavelength with the peak flux changes as

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s ≡σT 4

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λmax ≡2.898mm

T

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B(ν ,T) = const ×Tλ2 Rayleigh-Jean Approx.

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Luminosity = 4πa2 ×σTeff4 effective temperature

Astronomical magnitude system• Greek astronomer, Hipparchus (BC 2C) cataloged about 600 stars into 6

brightness bins. Later it was found that stars in the 6th bin (mag=6) are about 100 times fainter than stars in the first bin.

this means that 1 mag difference is roughly 2.52 times of difference in brightness.

From this fact, following formulae can be derived.

m: apparent magnitude, M: absolute magnitude (when a stars is at 10pc), F: flux, d: distance in parcsec, F0: zero magnitude flux

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2.525 ≈100

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mB −mA = 2.5log(FA /FB )m −M = 5logd −5m = 2.5log(F0 /F)

Absolute Magnitudes

• Absolute magnitude measured in a band-pass (MV, MB, etc.)• Absolute bolometric magnitude (Mbol = MV + BCV)o Sun, MV=4.83, Mbol=4.75

Names, Catalogs, URLs

Naming Stars• about 5000 stars can be seen by naked eyes in the whole skyo ~2500 stars at any given time (down to 6th mag)o about 50 stars that are very bright (m <~ 2nd mag)o We could name individual stars (e.g., Betelgeuse, Sirius, Antares, etc.)o effective up to few hundred stars

• Ptolemy (2nd century astronomer) “the brightest reddish star on the right shoulder”

• Al-Sufi (10th century, Persian)o “Armpit of the Central One” in Arabico “Bed Elgueze” in Latino “Betelgeuse in English

• Bayer Scheme: order stars in order of brightness in a given constellationo prefix: 24 Greek alphabetso suffix: 3 first letters from the Latin genitive constellation nameo e.g., Betelgeuse = α Ori = brightest star in Orion

Continue…o α CMa = the brightest star in Canis Major (big dog) = Siriuso since there are 88 constellations in the sky, at most we can name ~2000 stars this

way (88 x 24 alphabets = 2112)• Flamsteed scheme (18th century)o Why? with the advent of telescopes, there are too many stars.o within each constellation, number stars in order of increasing Right Ascension.o e.g., 58 Ori = Betelgeuse, Vega = 3 Lyro this scheme is still in use

How about variable stars?

Naming Variable Stars• in a given constellation, variable stars are named in order of discovery.o 1st discovered variable star in Ori R Orio R, S … Z, RR, RS … RZ, SS … SZ, TT … TZ, ZZ, AA … AZ, BB…BZ, … QZ (334 cases)o then, any more discoveries will be named by “V” + number + constellation. o e.g., RR Lyr, V 353 Ori, etc.

• Super Novao with a prefix “SN” followed by the discovered year, and discovered sequence.o A…Z, aa … az, ba … bz …o SN 1987A = the first discovered super nova in 1987

Stellar Catalogs• Durchmesterung numbers (≈300,000 stars, down to 9-10th mag)o based on photographic measurements of stars with telescopeso Bonner Durchmusterung = BD, northern hemishpereo Cordoba Durchmusterung = CD = CoD, southern hemisphereo within a given declination strip, number stars in order of increasing RAo e.g., Vega = BD +38 3238 3238th star in the declination strip +38 degrees.

• Henry Draper (HD) Catalogo not only name, brightness, and positions.o it also contains spectral type (e.g., temperature) info for 225,000 starso one of the most important catalog

Continue…• Subsequent major catalogso Hubble Guide Star Catalogs (GSC) : the need for good positions of many stars

down to m≈16th mag. about 15 million stars divided the whole sky into 9537 regions where each region contains roughly the

same number of stars efficient entry look up.o Hipparcos and Tycho catalogs European space satellite “Hipparcos” Hipparcos : measured distance, proper motions, brightness, and positions

(≈120,000 stars, down to m≈9th mag) Tycho : positions, proper motions, and brightness for (≈2.4 million stars down to

m≈11th mag).o USNO catalogs: extended version of Tycho USNO-B : ≈500 million stars UCAC-3 : ≈100 million stars, UCAC = USNO CCD Astrographic Catalog

o Other stellar catalogs: HR, SAO, FK5, Giclas, Gliese, CCDM, etc.

Catalogs of non-stellar sources• Catalogs of non-stellar sourceso Messier : 18th century, a catalog of 103 nebulae (galaxies, nearby interstellar

clouds, etc.)o late 19th century: New General Catalog (NGC) of 7840 nebulaeo e.g., Andromeda galaxy = M 31 = NGC 224

• Lots of other catalogs (at different wavelength regimes) currently there are ≈10,000 catalogs?? How can we navigate through these many catalogs??

Important astronomical websites• SIMBAD : http://simbad.u-strasbg.fr/simbado using a name of a star, get basic info (position, brightness, distance, etc.)

• Vizier : http://vizier.u-strasbg.fr/viz-bin/VizieRo can access ~10,000 astronomical catalogs

• Aladin : http://aladin.u-strasbg.fr/aladin.gmlo display image of a source o over-plotting catalog data on the image

• ADS : http://adsabs.harvard.edu/abstract_service.htmlo bibliographical database

In summary…

Important Concepts• Nomenclature of Stars• Stellar catalogs• Non-stellar catalogs

• Important websites

• Astronomical magnitude systemo apparent magnitudeo absolute magnitudeo bolometric magnitudeo Bolometric correction

Important Terms• Luminosity• Surface brightness• Stefan-Boltzmann Law• Wien’s displacement Law• Planck functiono Rayleigh-Jeans approximationo Effective Temperature

Chapter/sections covered in this lecture : Chapters 1 & 4