Download Star names and magnitudes

1B11
Foundations of Astronomy
Star names and magnitudes
Liz Puchnarewicz
[email protected]
www.ucl.ac.uk/webct
www.mssl.ucl.ac.uk/
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.
1B11 Star positions
The most important parameter you can know about any
astronomical source is its position on the sky.
Why?
1. Isolate, identify and re-visit the source
2. Check for transient sources, supernovae etc.
3. 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
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).
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.
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 >334.
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.
1B11 Example of a constellation
Constellation of Asterix
aAst
bAst
RRAst
eAst
NGC1234
gAst
dAstA
dAstB
M25
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
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.
1B11 More on the magnitude scale
For example, if star A has mv=1 and star B has mv=6
Their flux ratio,
flux (arbitrary units)
100
fA/fB = 2.512 mv(A)-mv(B)
= 2.5125
= 100
1
1
6
apparent visual magnitude, mv
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 mv=0, then:
mS-m0 = mS = 2.5log10(fm=0) - 2.5log10(fS)
=> mS = - 2.5log10(fS) + C
where C is a constant, ie C = 2.5log10(fm=0)
1B11 Magnitudes of different sources
-12.0
Full Moon
-5.0
Venus
-1.5
Sirius
0.0
4.5
Vega
Andromeda Galaxy
6.0
Naked eye limit
7.0
Neptune
14
Pluto
25
4m ground-based telescope limit
29
Hubble Space Telescope limit
1B11 How low can we go?
We can see stars as faint as mv=6.
The Hubble Space Telescope can reach mv=29.
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.
1B11 Magnitude systems
Flux (arbitrary units)
Every star has a different temperature => a different “colour”
Curves are
spectra for 3
stars, hot, Sunlike and cool.
4400 5500 7000
Wavelength
(Angstroms)
Measurements in different colour bands give different
magnitudes for different stars.
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.
transmission
(arbitrary units)
Harold Johnson (1921-1980) pioneered the standard UBV
system of filters for measuring magnitudes in various colours.
U
3600A
3000
B
4400A
4000
V
5500A
5000
6000
Wavelength
(Angstroms)
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.
Flux (arbitrary units)
1B11 Colour index
Every star has a
different
temperature => a
different “colour”
Wavelength
(Angstroms)
Star
B-V
V-R
blue
-ve
-ve
yellow
+ve
-ve
red
+ve
+ve
V-R
B
V
R
4400 5500 7000
B-V
1B11 Colour index
You can use many different combinations of colour index,
depending on the type of objects you’re looking at the science
you’re 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) B=0.73 V=0.96 => (B-V) = -0.23
Betelgeuse (aOri) B=2.66 V=0.96 => (B-V) = +1.86
So Spica is brighter in the blue, Betelgeuse is brighter in V.
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 yellowwhite
5000-6000
+0.5
Sun, Capella
K
Yellow-orange
3500-5000
+0.8
Arcturus,
Aldebaran
M
Red
<3500
+1.3
Betelgeuse,
Antares
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.
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 star’s 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
1B11 Absolute magnitudes
The flux from any source falls off as the inverse square of the
distance,
ie
intrinsic flux
observed flux 
2
distance
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):
Fm
10pc

2
FM
d
But from the
definition of
magnitudes,
m B  m A  Dm V  2.5 log 10
Definition of magnitudes
fA
fB
1B11 Distance modulus
So since
Then
mB  m A  Dm V  2.5 log 10
FM
m  M  2.5 log 10
Fm
fA
fB
2
 d   5 log d  5 log (10)
 2.5 log 10 
10
10
 10 
 m  M  5 log 10 d  5
where m-M is known as the distance modulus
The absolute (V band) magnitude of the Sun is +4.6.
Definition of magnitudes
1B11 Absolute bolometric magnitude
So
mbol  2.5 log 10 Fbol  C
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  (mbol  mV )  (Mbol  MV )
 FV 

 2.5 log 10 
 Fbol 
Mbol  MV  BC
BC is always negative
and is determined
empirically.
Definition of magnitudes
1B11 Reddening and extinction
reddening
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.
1B11 Reddening and colour excess
Extinction is hard to measure because it is monochromatic,
but scattering is easier because its effect is wavelengthdependent 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
It’s measured in magnitudes – and it’s always positive! (why?)
Colour index
1B11 Correcting for reddening and extinction
Spectral features
(absorption and emission
lines, red continuum)
Spectral type
Intrinsic continuum shape
Generally, extinction is given by:
Calculate (B – V)0
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)
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:
m0 – M = 5log10d –5
From A = m – m0,
=> m0 = m – A
(m – M) = 5log10d – 5 + A
Distance modulus
Flux fl (erg cm-2 s-1 A-1)
1B11 Bolometric luminosity
Stellar
spectrum
V
Wavelength, l
(Angstroms)
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).
Fbol = integ(0-inf)Flambda dl
where Fl is the flux at each wavelength (l) in the spectrum.
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