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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

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Concepts from ASTR1010 & Names, Catalogs, URLs

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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