Concepts from ASTR1010 &amp; Names, Catalogs, URLs

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Concepts from ASTR1010 &amp; Names, Catalogs, URLs. ASTR 3010 Lecture 2 Chapter 1 &amp; 4. Luminosity, flux density, and surface brightness . Luminosity = Total energy emitted by the source per unit time (ergs/sec) independent on the distance to the source

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### 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)
• independent on the distance to the source
• Lsun = 3.825×1026 W = 3.825×1033 ergs/sec
• Apparent Brightness = luminosity/area
• Flux density of the Sun at the Earth position

1,370 W/m2 = Solar constant

• Surface brightness = flux density at the surface of the source over 1 steradian
• For a given blackbody of temperature T, the surface brightness is

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

effective temperature

Rayleigh-Jean Approx.

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

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

### Names, Catalogs, URLs

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

“the brightest reddish star on the right shoulder”

• Al-Sufi (10th century, Persian)
• “Armpit of the Central One” in Arabic
• “Bed Elgueze” in Latin
• “Betelgeuse in English
• Bayer Scheme: order stars in order of brightness in a given constellation
• prefix: 24 Greek alphabets
• suffix: 3 first letters from the Latin genitive constellation name
• e.g., Betelgeuse = αOri = brightest star in Orion
Continue…
• αCMa = the brightest star in Canis Major (big dog) = Sirius
• 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)
• Why? with the advent of telescopes, there are too many stars.
• within each constellation, number stars in order of increasing Right Ascension.
• e.g., 58 Ori = Betelgeuse, Vega = 3 Lyr
• this scheme is still in use

Naming Variable Stars
• in a given constellation, variable stars are named in order of discovery.
• 1st discovered variable star in Ori R Ori
• R, S … Z, RR, RS … RZ, SS … SZ, TT … TZ, ZZ, AA … AZ, BB…BZ, … QZ (334 cases)
• then, any more discoveries will be named by “V” + number + constellation.
• e.g., RR Lyr, V 353 Ori, etc.
• Super Nova
• with a prefix “SN” followed by the discovered year, and discovered sequence.
• A…Z, aa … az, ba … bz …
• SN 1987A = the first discovered super nova in 1987
Stellar Catalogs
• Durchmesterung numbers (≈300,000 stars, down to 9-10thmag)
• based on photographic measurements of stars with telescopes
• Bonner Durchmusterung = BD, northern hemishpere
• Cordoba Durchmusterung = CD = CoD, southern hemisphere
• within a given declination strip, number stars in order of increasing RA
• e.g., Vega = BD +38 3238  3238th star in the declination strip +38 degrees.
• Henry Draper (HD) Catalog
• not only name, brightness, and positions.
• it also contains spectral type (e.g., temperature) info for 225,000 stars
• one of the most important catalog
Continue…
• Subsequent major catalogs
• Hubble Guide Star Catalogs (GSC) : the need for good positions of many stars down to m≈16th mag.
• divided the whole sky into 9537 regions where each region contains roughly the same number of stars  efficient entry look up.
• Hipparcos and Tycho catalogs  European space satellite “Hipparcos”
• Hipparcos : measured distance, proper motions, brightness, and positions (≈120,000 stars, down to m≈9thmag)
• Tycho : positions, proper motions, and brightness for (≈2.4 million stars down to m≈11thmag).
• USNO catalogs: extended version of Tycho
• USNO-B : ≈500 million stars
• UCAC-3 : ≈100 million stars, UCAC = USNO CCD Astrographic Catalog
• Other stellar catalogs: HR, SAO, FK5, Giclas, Gliese, CCDM, etc.
Catalogs of non-stellar sources
• Catalogs of non-stellar sources
• Messier : 18th century, a catalog of 103 nebulae (galaxies, nearby interstellar clouds, etc.)
• late 19th century: New General Catalog (NGC) of 7840 nebulae
• 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
• using a name of a star, get basic info (position, brightness, distance, etc.)
• Vizier : http://vizier.u-strasbg.fr/viz-bin/VizieR
• can access ~10,000 astronomical catalogs
• display image of a source
• over-plotting catalog data on the image
• bibliographical database
In summary…

Important Concepts

Important Terms

Luminosity

Surface brightness

Stefan-Boltzmann Law

Wien’s displacement Law

Planck function

Rayleigh-Jeans approximation

Effective Temperature

• Nomenclature of Stars
• Stellar catalogs
• Non-stellar catalogs
• Important websites
• Astronomical magnitude system
• apparent magnitude
• absolute magnitude
• bolometric magnitude
• Bolometric correction
• Chapter/sections covered in this lecture : Chapters 1 & 4