1B11 Foundations of Astronomy Star names and magnitudes

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1B11 Foundations of AstronomyStar names and
magnitudes

• Liz Puchnarewicz
• emp_at_mssl.ucl.ac.uk
• www.ucl.ac.uk/webct
• www.mssl.ucl.ac.uk/

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1B11 Our night sky
Our Sun is one of approximately 200 billion stars
in our Galaxy, the Milky Way. It lies in one of
the spiral arms, about two-thirds of the way out
from the centre, which is called Sagittarius A
On a clear dark night in the UK, we can see 3000
distinct stars, plus the fuzzy glow from the
plane of the Milky Way.
From the very earliest times, humans have grouped
patterns of stars into constellations, often
animals and characters from myths and legends.
There are now 88 official constellations.
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1B11 Star positions

• The most important parameter you can know about
  any astronomical source is its position on the
  sky.
• Why?
• Isolate, identify and re-visit the source
• Check for transient sources, supernovae etc.
• Associate sources at different wavelengths
• By grouping stars into constellations, our
  ancestors developed the first system for
  unambiguously identifying celestial sources. Now,
  we use co-ordinate systems based on angular
  distance scales.

Astronomical co-ordinates
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1B11 Constellation names
Constellations, their names and boundaries were
defined by the International Astronomical Union
(IAC) in 1930. The brightest stars have their own
names, eg Orion, Vega, Aldebaran, Polaris,
Betelgeuse. Many naked-eye objects are
identified by their constellation name
abbreviated to an IAU 3-letter standard, followed
by a letter from the Greek alphabet in order of
decreasing brightness (eg, Sirius,the brightest
star in Canis Majoris, is also known as aCMa).
This was devised by Bayer in 1603. If there are
more than 24 stars in a constellation, then the
remainder are numbered in order of Right
Ascension (Flamsteed, 1925).
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1B11 Faint stars and catalogues
Fainter stars Some stars which are too faint to
be seen with the naked eye are identified by a
catalogue number, eg HD Harvard Revised HR
Henry Draper eg Sirius aCMa HR2491
HD48915 These catalogues give information on the
position, colour, brightness and type of
stars. There are now many, many different types
of astronomical catalogue. VizieR is a web
service run by the CDS which is effectively a
catalogue of catalogues.
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1B11 Multiple and variable stars
Multiple stars Most stars in the Galaxy (at least
a half) are in binary and multiple systems. In
these cases, components are labelled A, B, C.
etc. in order of decreasing brightness. Eg. 61Cyg
is a double star the brighter component is
61CygA, the fainter one is 61CygB. Variable
stars Many stars are variable, which complicates
labelling based on brightness! If they have a
Bayer designation (eg dCep), they keep these.
Otherwise, their constellation name is prefixed
by one or two letters, depending on the time of
discovery. 334 combinations of letters are
available after that they are prefixed by Vnnn
where nnn is gt334.
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1B11 Nebulae and galaxies
Non-stellar objects In the late 18th Century,
Charles Messier compiled a list of about 100
diffuse objects, to distinguish these from
comets. This has become known as a collection of
beautiful deep sky objects, including galaxies,
nebulae and clusters of stars. All 110 Messier
objects can be seen at the SEDS Messier
Database. Another catalogue of fuzzy objects, the
New General Catalogue, was compiled in 1888 and
contains 7840 objects including galaxies, star
clusters, planetary nebulae and supernova
remnants.
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1B11 Example of a constellation
Constellation of Asterix
aAst
bAst
RRAst
eAst
gAst
NGC1234
dAstA
M25
dAstB
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1B11 The magnitude scale
Hipparchus (120BC) and Ptolemy (180AD) devised
the magnitude scale for measuring stellar
brightness, based on the response of the eye,
which is logarithmic.
brightest stars are 1st magnitude
faintest stars are 6th magnitude
this star
this star
is 100 times brighter than Pogson (1856)
A difference in 1 magnitude a factor of 2.512
in brightness
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1B11 Defining magnitudes
Thus Pogson defined the magnitude scale for
brightness. This is the brightness that a star
appears to have on the sky, thus it is referred
to as apparent magnitude. Also this is the
brightness as it appears in our eyes. Our eyes
have their own response to light, ie they act as
a kind of filter, sensitive over a certain
wavelength range. This filter is called the
visual band and is centred on 5500
Angstroms. Thus, strictly speaking, these are
apparent visual magnitudes, mv.
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1B11 More on the magnitude scale
For example, if star A has mv1 and star B has
mv6
Their flux ratio, fA/fB 2.512 mv(A)-mv(B)
2.5125 100
100
flux (arbitrary units)
1
1
6
apparent visual magnitude, mv
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1B11 Converting from fluxes to magnitudes
So if you know the magnitudes of two stars, you
can calculate the ratio of their fluxes using
fA/fB 2.512 mv(A)-mv(B) Conversely, if you know
their flux ratio, you can calculate the
difference in magnitudes since mB-mA Dmv
2.5log10(fA/fB). To calculate the apparent visual
magnitude itself, you need to know the apparent
visual flux for an object at mv0, then mS-m0
mS 2.5log10(fm0) - 2.5log10(fS) gt mS -
2.5log10(fS) C where C is a constant, ie C
2.5log10(fm0)
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1B11 Magnitudes of different sources
-12.0 Full Moon
-5.0 Venus
-1.5 Sirius
0.0 Vega
4.5 Andromeda Galaxy
6.0 Naked eye limit
7.0 Neptune
14 Pluto
25 4m ground-based telescope limit
29 Hubble Space Telescope limit
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1B11 How low can we go?
We can see stars as faint as mv6. The Hubble
Space Telescope can reach mv29. How much fainter
is this? FHST/Feye 100(mHST-meye)/5
10(mHST-meye)/2.5 109.2 In other words, HST can
see stars which are over a billion times fainter
than we can see with the naked eye.
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1B11 Magnitude systems
Every star has a different temperature gt a
different colour
Curves are spectra for 3 stars, hot, Sun-like and
cool.
Flux (arbitrary units)
Wavelength (Angstroms)
4400 5500 7000
Measurements in different colour bands give
different magnitudes for different stars.
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1B11 UBV Johnson System
Different types of stars emit strongly at
different wavelengths, thus will have different
strengths depending on the filter used to observe
them. Harold Johnson (1921-1980) pioneered the
standard UBV system of filters for measuring
magnitudes in various colours.
transmission (arbitrary units)
U 3600A
B 4400A
V 5500A
Wavelength (Angstroms)
3000 4000 5000
6000
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1B11 UBV Johnson System
In the Johnston UBV system, each filter is about
1000A wide.
Filter name band Apparent magnitude Central wavelength
ultraviolet U mu 3600
blue B mb 4400
visible V mv 5500
red R mr 7000
infra-red I mi 8000
(other colour filter systems are available!)
The system was extended to R and I, then J,H,K in
the IR.
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1B11 Colour index
Every star has a different temperature gt a
different colour
Flux (arbitrary units)
Wavelength (Angstroms)
B V R
4400 5500 7000
Star B-V V-R
blue -ve -ve
yellow ve -ve
red ve ve
V-R
B-V
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1B11 Colour index
You can use many different combinations of colour
index, depending on the type of objects youre
looking at the science youre interested in. eg.,
(U-B), (B-V) for hot stars is useful (V-R), (R-I)
for red stars or for red properties of source
population For example Spica (aVir) B0.73
V0.96 gt (B-V) -0.23 Betelgeuse (aOri)
B2.66 V0.96 gt (B-V) 1.86 So Spica is
brighter in the blue, Betelgeuse is brighter in V.
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1B11 Temperatures, colours and classification
Spectral class Colour Surface temp (K) B-V example
O Blue-white 30000-35000 -0.4 Naos
B Blue-white 11000-30000 -0.2 Rigel, Spica
A Blue-white 7500-11000 0.0 Sirius, Vega
F Blue-white to white 6000-7500 0.3 Canopus, Procyon
G White to yellow-white 5000-6000 0.5 Sun, Capella
K Yellow-orange 3500-5000 0.8 Arcturus, Aldebaran
M Red lt3500 1.3 Betelgeuse, Antares
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1B11 Temperatures, colours and classification
And thereafter, stars are further subclassified
using the numbers 0-9 O B A F G K M R N
S 0,1,2,3,4,5,6,7,8,9 For example, our Sun is a
G2 type star and has a temperature of 5800K.
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1B11 Absolute magnitudes
Knowing how bright a star is on the sky is very
useful but the stars all lie at very different
distances from the Earth. Scientifically, we want
to know a stars intrinsic flux ie its
luminosity. Astronomers have two ways of
quantifying this Absolute magnitudes and
Luminosities
The absolute magnitude is the magnitude a star
would have if it were placed 10 parsecs away from
the Earth.
10pc
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1B11 Absolute magnitudes
The flux from any source falls off as the inverse
square of the distance, ie Example a star lies
at distance d with apparent magnitude m and flux
Fm. If this star was 10 parsecs away so that the
flux was FM, then (because of the inverse square
law) But from the definition of magnitudes,
Definition of magnitudes
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1B11 Distance modulus
So since
where m-M is known as the distance modulus
The absolute (V band) magnitude of the Sun is
4.6.
Definition of magnitudes
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1B11 Absolute bolometric magnitude
So
Similarly, we can define an absolute bolometric
magnitude, Mbol (ie mbol at 10 pc). Visual
magnitudes can be converted to bolometric
magnitudes via the bolometric correction, BC
BC is always negative and is determined
empirically.
Definition of magnitudes
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1B11 Reddening and extinction
Any dust which lies between an observer and a
source will absorb light from the source and use
it to heat the dust. This is extinction. Dust
will also scatter the light and blue light is
scattered more than red, which makes the source
look more red (although strictly speaking, less
blue) and this is called reddening. This effect
makes sunsets and sunrises red.
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1B11 Reddening and colour excess
Extinction is hard to measure because it is
monochromatic, but scattering is easier because
its effect is wavelength-dependent so will
manifest itself as a colour change. Observed
colour (B V) Intrinsic colour (B
V)0 Reddening is measured by the colour excess
which is defined as E(B V) (B V) (B
V)0 Its measured in magnitudes and its always
positive! (why?)
Colour index
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1B11 Correcting for reddening and extinction
Spectral features (absorption and emission lines,
red continuum)
Spectral type
Intrinsic continuum shape
Calculate (B V)0
Generally, extinction is given by A m
m0 Where m0 would be the apparent magnitude if
there was no extinction.
In the V band AV V V0 3.1E(B V)
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1B11 Including extinction in distance modulus
Remember the distance modulus equation
m M 5log10d 5
Strictly speaking, m in this equation has been
assumed to be unaffected by dust, so it should
read From A m m0, gt m0 m A
m0 M 5log10d 5
(m M) 5log10d 5 A
Distance modulus
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1B11 Bolometric luminosity
Stellar spectrum
Filter band magnitudes (eg U, B, V) will only
give the flux at particular wavelengths (shaded).
For the total bolometric luminosity, need to
integrate over all wavelengths (pink).
Flux fl (erg cm-2 s-1 A-1)
Wavelength, l (Angstroms)
V
Fbol integ(0-inf)Flambda dl
where Fl is the flux at each wavelength (l) in
the spectrum.
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1B11 Bolometric magnitudes
Alternatively, the total intrinsic emission of a
star integrated over all wavelengths may be
expressed as a bolometric magnitude, Mbol. Since
mB-mA Dmv 2.5log10(fA/fB) Mbol(Sun)
Mbol(star) 2.5log10(Lstar/LSun) Mbol(Sun)
4.75 Log10(Lstar/LSun) 1.90 0.4Mbol(star)
Converting fluxes to magnitudes