Transcript lec-galatic-observat..

Swinburne Online Education Exploring Galaxies and the Cosmos
The Milky Way: General Structure
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Activity:
The Milky Way star clouds of Sagittarius
in the direction of our Galaxy’s centre
© Swinburne University of Technology
Observing the Milky
Way
Summary
1. To assess our Milky Way Galaxy from our location
within it.
2. To use the knowledge (of only this century) that our
Galaxy’s properties may be anticipated from, and
compared with, properties of galaxies outside our own.
3. A final visual appreciation
of our Galaxy before we rely
on Observatory images.
Title screen image: The wide, bright, central
(Sagittarius) region of our Galaxy. Although
finer detail photos are available, the author
included his own photo here as an example
of a standard SLR camera image. 60 sec
guided exposure on 1600 film.
Introduction
Galaxy studies take us from naked eye familiarity to
images seen only by the world’s largest telescopes.
This is our last chance (as we move out into the wider
Universe) to consider some naked eye views of the
night sky.
Naked eye views of the Milky Way
To any night sky observer, especially away from city lights,
the normal random distribution of stars is broken by the band
of light (and increased star density) we call the Milky Way.
Casual observations of the Milky Way at different times and
dates give confusingly different orientations of its band of light
relative to the horizon, yet it has a simple overall general
‘shape’.
Let’s look at some views of the Milky Way to discover its full
extent.
count falls off with
lky Way near Star
Southern
Cross
distance away from
In this image, a torrent of
stars cascades from the
Southern Cross down to
the horizon - the Milky
Way.
Note the dark ‘Coal Sack’
area below a Crucis.
Note other dark features
Southeast, 10pm, early March,
from southern Australia
a Centauri, the closest star
This is a tree
the Milky Way
The tree and the Coal Sack have something in common - they are both foreground features
Follow this link for background material on estimating
the distance to a Centauri
The Milky Way near Scorpius
Here we use
computer software to
show Scorpius and
Sagittarius rising
above the eastern
horizon in May from
southern Australia.
Note the grey contours
(isophotes) indicating
the brightness of the
Milky Way.
50o field of view
This region is the brightest, widest mix of stars and
glow of the Milky Way.
The Milky Way near Orion
The western horizon,
with Sirius, through
Orion to the Hyades
and Pleiades clusters,
setting in April from
Southern USA.
Again, note the grey
isophotes of the Milky
Way.
80o field of view
All Sky Milky Way views
All sky (fish eye) view showing
the Milky Way arching across
the sky (centred by the bright
Sagittarius region).
At a different time the fainter
half of the Milky Way (centred
by the Orion region) completes
an evident ‘ring’.
n
Antares
Sagittarius,
Scorpius region
LMC
SMC
At the latitude
of Sydney,
Australia, just
before
midnight in
early October,
the Milky Way
lies entirely
around the
horizon - a
great circle on
the celestial
sphere.
Orion region
The Milky Way thus has the simple overall general
shape of a great circle ring of faint light and increased
star density, with dark features and an increased
brightness and bulk in the direction of Sagittarius.
We observe from our particular location, on a tilted
Earth, orbiting the Sun in a different plane to that of the
Milky Way. No wonder its different orientations in the
sky can be initially confusing.
Activity: Observe the Milky Way’s orientation and
extent over a few hours.
Alternative Structures
The ring of bright and faint stars we see as the Milky
Way could either be a real ring or a disk of stars, with
the Sun at or near the centre.
Measurement of a wide range of distances to stars
established that we are looking out through a disk of
stars, like ants looking out at salt grains scattered over a
transparent plate.
Follow this link to background material
on Magnitudes and Distances
Historical: The extent of the ‘Universe’.
At the start of this century, all extended regions of
misty light were called nebulae (latin - ‘clouds’).
Some of these evidently
were associated with star
clusters whose distance
could be estimated.
Were all of these nebulae within
our Milky Way ‘Universe’? including those with ‘spiral’
features and detail unresolved
by telescopes of the day?
The Magellanic Clouds
The naked eye Large and Small
Magellanic Clouds never set for
observers south of latitude 25o S.
LMC
They are 33o and 45o (respectively)
from the plane of the Milky Way. (See
previous Map). They must have hinted
at the fact - now known - that they are
SMC
star systems, external to, but gravitationally bound to, the Milky Way.
47Tuc
The properties of Cepheid variables in
globular
cluster
the LMC and SMC lead to their use as
distance indicators for work by
Author’s photo to show close-to-naked-eye view
Hubble and Shapley.
in relation to horizon. 35o field, 50mm camera
lens, 400ASA, 5 minute guided exposure.
Follow this link to
background material on using Cepheid variables
The Milky Way - One galaxy amongst many.
With the 100” telescope in 1924, Edwin Hubble identified
Cepheid variable stars in the ‘Great Nebula in Andromeda’
(M31*) showing it to be far outside our own star system.
The term ‘galaxy’ now means an isolated assembly of
millions (to billions) of stars, gas and dust. The Milky Way
comprises our own Galaxy (deserving capital ‘G”).
Which type of galaxy is our Milky Way, from the
evidence from our view from within it?
?
?
?
Viewing a spiral galaxy face-on.
Can we match the bright centre and spiral features of a
galaxy like this to what we see in our own Galaxy?
AAT 008
M83 is some 20 million light
years away and appears at
magnitude 7.6 in Hydra. Its
apparent size in the sky is
about one-third that of the
Moon.
Note more distant galaxy
Foreground stars in our own Galaxy
Does our Galaxy have a bright central
region (and can we see it)?
Historical: Was the Sun near the Centre of
the Milky Way system?
In 1917 Harlow Shapley investigated the distribution of
globular clusters* - which appear in an apparent
spherical halo above and below the disk of the Milky
Way.
The Sun was not near the
centre of the distribution.
Sagittarius
A third of the Milky Way’s
globulars (some 150 are now
known) were in fact in the
direction of Sagittarius.
Expected to share the same
centre of mass as the globulars,
the galactic disk was therefore
not centred by the Sun.
*Click here to find out about globular clusters
A side-on view of a galaxy.
AAT 023
NGC253 is a 7th magnitude
galaxy in Sculptor. It is like
M83 but suggestively sideon. Imagine our Sun is one
of those billions of stars, at
the location shown.
Sweeping around our new
celestial sphere, from the location
shown, we would expect to see a
great circle of higher star concentration; brighter toward the galactic Away from the plane of the galaxy,
centre or a nearby spiral arm - for we would expect fewer stars - as,
indeed, is the case.
us, the Sagittarius region.
Galactic Gridlines Introduction
In the next frame we are going to look at another
region of the Milky Way - around Aquila.
We will also use that frame to add two great circles with
which you are already familiar:
- the celestial equator dividing the celestial sphere,
- the ecliptic near which, solar system objects are found.
Corresponding approximately to the circle of the Milky
Way, another great circle - the Galactic Equator - will
also be shown.
Try to get a feel for the angles between
these three important planes.
lky Way near Aquila
Galactic Equator
Celestial Equator
Ecliptic
All great circles (including the horizon) are shown with 30o tick intervals
From Earth’s Equator, looking east at 10pm mid June,
the Milky Way at Aquila is divided by a dark rift.
Historical: Was the Milky Way really a
disk of stars?
Late last century, a disk theory was nearly abandoned
before the improvement in telescopes and photography.
It was thought that the dark rifts (as in the Aquila frame) in the
Milky Way and dark patches (such as the Coal Sack) indicated an
absence of stars and that, if within a disk arrangement, it would be
too coincidental that they lined up, like tunnels, with our (chance)
line of sight.
The discovery of the presence of gas and dust causing dark
nebulae and equatorial rifts, and the visibility of similar dark lanes
in other side-on galaxies (as in NGC253) revived the disk concept;
not a uniform disk, but one with a central bulge and spiral arms.
alactic Equator
17h
Note: All
coordinate
values
increase
eastwards toward the
eastern
horizon
270
19h
300
NE
60
21h
East
120
SE
Working again with the Aquila region, we now
add coordinates to the great circles shown ...
Click for: Horizon
Ecliptic
Celestial Equator (2hrs = 30o)
Galactic Equator X = centre
The galactic coordinate grid
Galactic longitude (l) is measured in degrees eastwards
around the galactic equator from 0o in Sagittarius.
N (Coma Berenices)
Galactic latitude (b) is measured
in degrees North (+ve) and South
(-ve) of the galactic equator.
Well away from the plane of the Milky
Way we have views, clear of stars
(and gas and dust), to distant
galaxies and clusters of galaxies
- for example in Coma Berenices
and Virgo (near the North Galactic
Pole) and Sculptor and Fornax (near
the South Galactic Pole).
l
b
S (Sculptor)
From AAT047 image
M20 Trifid
M8 Lagoon
Galactic longitude 0o,
a ‘spout’s’ length off
the ‘teapot’
M16 Eagle
Nebula
Hubble Space
Telescope Image
M17 Swan
Here are some of
the notable features
in the direction of
the Galactic Centre
Note the dark material
(but not dark matter!)
hiding our view through
to the Galactic centre
X
Sagittarius
(teapot)
M7 M6
(naked eye
cluster)
Tail of
Scorpius
Use arrow keys to step back and forth to revise
Summary
There are three broad regions of our Galaxy:
A disk, thin compared with its lateral extent,
A brighter, wider central region or bulge,
A halo of globular clusters (and, as we will see, stars and
possibly dark matter.)
The next Activity will look in detail at the type and
distribution of stars in these regions - and the spiral
structure must wait for later Activities.
The next frame shows the dimensions of the Galaxy that
our next Activity will reinforce.
The dimensions of our Galaxy.
The Sun is located about 8000 parsecs* (8 Kpc) from
the centre. Here we examine a side-on view:
.
.
Spherical halo Globular clusters, stars, dark matter (?)
.
.
.
.
.
.
.
.
.
Thin disk . . 8 Kpc 2 Kpc
.
0.6 Kpc
.
.
.
.
Central bulge
50 Kpc
.
..
.
.
The disk and bulge contain at least 100 billion stars
.
Exercise: Review these dimensions in light years, given: 1 pc = 3.26 ly
*(Remember: 1 parsec = 30.86 million million km)
Image Credits
AAT © David Malin (used with permission):
M83
AAT028
NGC253 AAT003
http://www.aao.gov.au/local/www/dfm
Individual AAT images, © David Malin (used with permission), shown
with a 6 character code - such as AAT028 - are found at the website
ending with that code; eg:
http://www.aao.gov.au/local/www/dfm/aat028.html
© the Author:
Southern Cross region
Sagittarius region
Magellanic Clouds
http://www.gsat.edu.au/astronet
Use of output from the program GUIDE 7.0, courtesy of its author:
http://www.projectpluto.com
In the next Activity, we will look at the range in luminosity
of stars in our region; the existence of two broad
Populations of stars and how this, and the existence of
interstellar extinction, confused early estimates of the
size of our Galaxy and the distance to other galaxies.
Hit the Esc key (escape)
to return to the Index Page
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Revisiting the parsec
What is the significance of the distance unit called the
parsec?
Viewing the Earth’s orbit face-on from a star one parsec
away, the Earth-Sun separation* would be 1 arcsecond* (1”).
*Click here to find out about arcsec (”)
Note: Diagrams such as this give a false impression of ‘nearness’ of
stars. The length of the triangle should be 206,265 times the width!
* 1 astronomical unit (AU) = 149,597,870 km.
1 parsec = 30.86 million million km!
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Distance determination (i) - by trigonometry.
No star is as close as one parsec (pc). a Centauri is 1.3 pc.
d
a
a
a
We can measure the same (parallax) angle a which the
star appears to move against background stars, for two
dates, 3 months apart, in our orbit around the Sun.
A star at double the distance would halve the parallax
angle. In general d (parsecs) =1 / a (arcseconds)
Note: The tiny angle (a=0.76”) between Sun and Earth from even the
closest star (a Cen) emphasises the difficulty in detecting planets
which may be over 20 magnitudes fainter than their parent star.
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The small angle formula.
A frequently used formula in astronomy applications.
The distance around a full circle of
radius r for a sweep angle of 360o is 2pr.
s
a
The arc length s is the same part of
r
the full circle 2pr as the angle a is to
360
ie: s/2pr = a/360
or: s = ra p/180 when a is in degrees
or: s = ra when a is in radians
or: s = ra / 206265 when a is in arcseconds
For small angles, this also gives the straight line (chord)
length.
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Parsecs and Light years.
The relation between the two distance units.
By definition of a parsec, in the formula s = ra” / 206265,
a=1” and s=1 Astronomical Unit (149,597,870 km).
So one parsec (r) = 30.86 million million km.
One light year is the distance light travels in a year
at 300,000 km per second.
Multiply by 60x60x24x365 for a year, giving
one light year = 9.46 million million km.
So 1 parsec = 3.26 light years and
1 light year = 0.31 parsecs.
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a
Small angle formula applications.
Using: s = ra / 206265 with a in arcseconds
r
1. What is the actual distance (r) to a Centauri?
Parallax a=0.76”,
Earth-Sun baseline s = 150 million km,
so distance r = 1.5x108 x 206265 / 0.76
= ~41 million million km.
2. What is the angular size (a) of a Centauri,
assuming it is a star of similar size to the
Sun? (s = 1.4x106km)
s
s
r
a
a=1.4x106 x 206265 / 4.1x1013 = 0.007” !
Note: That’s why we don’t see ‘size’ in stars.
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Apparent magnitude and brightness.
By its modern definition a difference of five magnitudes
implies a brightness factor of exactly 100.
ie: for m2 - m1 = 5, b1/b2 = 100 or 102
In general, b1/b2 = 10 2/5 (m2
- m1 )
Then: log* b2/b1 = 0.4(m1-m2)
or 10 0.4(m2
or:
1
2
-m )
1
m1-m2 = 2.5 log b2/b1
The globular cluster w Cen, of >100,000 stars, appears
as magnitude 3.5. To a first approximation (similar stars,
not masking others), what is their individual magnitude?
Answer: m1 - 3.5 = 2.5 log 105 = 2.5x5
so m1 = 16
10x
Note: The log key is provided on scientific calculators
*Click here to find out about logarithms
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Brightness and distance.
How would a star’s brightness (and magnitude) change if
we, say, doubled its distance?
A small surface area (such as a
telescope mirror) collecting light from
a star, is just a part of the surface
area 4pr2 of the sphere of radius r.
r
With changing distance the light
falling on the same collecting area
will therefore change inversely with
the square of the distance to the star.
This is the inverse square law. So doubling the distance would quarter
the brightness - a magnitude change of m1-m2=2.5log4 or 1.5
background
Absolute Magnitude.
The inverse square law allows us to recalculate a star’s
brightness if the star was at a different distance.
If we could consider all stars at some fixed distance, we
could compare their real brightnesses. The fixed distance
is chosen to be 10 parsecs. A star at distance d with
apparent magnitude m, will, if moved to distance 10
parsecs, appear at what we call its absolute magnitude M.
inverse square law
m - M = 2.5 log b10/bd = 2.5 log (d/10)2 = 5 log d - 5
m-M
is called the distance modulus.
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Absolute Magnitude example.
Compare the real intensity of Sirius with that of Rigel.
Using m - M = 5 log d - 5
The brightest star Sirius, with m=-1.4 is only 8.6 light
years away. Rigel appears fainter at magnitude 0.2 but is
770 light years away. (Apply 1 parsec=3.26 light years)
What are their true relative brightnesses?
Sirius: M=-1.4-5log8.6/3.26+5 =1.5
Rigel: M=0.2-5log770/3.26+5=-6.6
bRigel/bSirius =10 0.4(1.5-(-6.6)) = 1,738 times brighter!
Absolute magnitude can be inferred from a star’s intrinsic
properties.
Distances obtained from trigonometric parallaxes are useful to about
500 parsecs - and now improved by Hipparchus satellite measures.
We can discover the brightness of a light bulb
from the wattage written on it! A star gives away
its absolute Magnitude by its colour or spectrum
characteristics - its place in the H-R diagram.
Absolute
Magnitude
background
Distance determination - ii).
COLOUR
Observing a star’s apparent magnitude and
colour, and deriving M, leads to its distance,
using m - M = 5 log d - 5
This is called the spectroscopic parallax method of distance
determination and may be accurate only to within about 50%.
Stars in a cluster add weight to its distance determination.
The non-evolved portion of cluster stars in
the H-R diagram should fit the line of the
main sequence.
The apparent magnitude of the cluster is,
of course, much fainter.
magnitude
background
Distance determination - iii).
M
m
COLOUR
The correction to fit the main sequence is
the distance modulus m - M giving
5log d - 5 and hence the cluster distance.
This distance measurement method is
called main sequence fitting - a variation
of spectroscopic parallax for individual
stars.
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Cepheid variable stars.
The wonderful discovery, by Henrietta Leavitt in 1907, that the
brighter variable stars in the Large Magellanic Cloud* (of the
same type as d Cephei) had the longer periods, provided a
method to measure distances to galaxies millions of light years
away - provided these very luminous stars could be detected.
A Cepheid variable’s observed period
leads to its absolute Magnitude which,
with its known apparent magnitude,
leads to its distance using
log d=(m-M+5)/5
Absolute magnitude
0
5
-5
-10
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Distance determination - iv).
1
10
100
Period in days
*The LMC is over 160,000 light years away, so relative
brightness differences in its stars are true differences.
Henrietta Leavitt subsequently confirmed the
relationship for Cepheids in the Small Magellanic
Cloud.
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Arcsec
Arcsec (symbol “) is short for
seconds of arc and is a measure of
angle.
A full circle is defined as an angle of 360 degrees.
Each degree (o) includes 60 arc minutes (‘).
Each arc minute (‘) includes 60 arc seconds (arcsec, “).
So one arcsec is one-sixtieth of one-sixtieth of
one-three-hundred-and-sixtieth of a circle …
a very small slice or the circle indeed.
There are 1 296 000 arcsec (“) in a full circle.
Take 1/360th of a circle
Take 1/60th of that
Take 1/60th of that
No, you can’t see an arcsec in a diagram like this: it’s much too small a slice!
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Logarithmic scales
A logarithmic view in mathematics uses a
different way of making sequences of numbers.
Instead of looking at what happens when you
increase from 1 to 2 to 3, you consider instead
what happens when you go from 1 to 10 to 100.
In the 1, 2, 3 view, the increase is made by
adding. The result is an arithmetic
progression. Here’s another one.
In the 1, 10, 100 view, the increase is made by
multiplying. The result is a geometric
progression.
The second way of increasing can help a lot
when drawing graphs. Here’s a sample.
Arithmetic
2 4 8 10
6
4
12
36
102
306
Geometric
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An example
Why on Earth (or off Earth, in
the case of your studies)
would anyone want to use a
logarithmic method?
Let’s consider our tame
astronomer who wants to
illustrate the amount of
money he owns (or hopes to
own) at particular ages of his
life.
Age
Money
1
$1.00
5
$10.00
10
$50.00
50
$10,000.00
100 $1,000,000.00
Trouble is, when he draws a graph of Money versus
Age, the detail near the beginning isn’t clear.
This is because the
vertical scale has to
go up to $100,000,
so amounts like $1,
$5 and $10 are too
tiny to show.
120000
100000
80000
Money ($)
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Not much good
60000
40000
20000
0
-20000 1
5
10
Age (years)
50
100
The solution is to draw the graph again, but this time he
uses a logarithmic vertical scale where you increase by
multiplying by 10 rather than by adding $10,000.
A scale that goes up
by multiplying rather
100000
than by adding is
10000
called a logarithmic
scale.
1000
It is particularly
100
useful when drawing
graphs in astronomy,
10
where figures can
vary so very widely.
1
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1
5
10
50
to the Activity
Age (years)
Money ($)
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The solution
100
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Activity
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Globular Clusters
Globular Clusters are dense concentrations of stars
- up to a million of them
bound together by their mutual gravity
Their total mass is so large
that gravity pulls the cluster
into the shape of a sphere
… or globule
… hence the name
GLOBULAR clusters
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What if we lived in a Globular Cluster?
If the Sun were to have evolved within a
Globular Cluster we would be surrounded by
thousands of stars within the nearest parsec
alone.
That’s a far cry from the zero stars that share
the cubic parsec surrounding our Sun today.
The sky would be so bright with stars at night,
assuming there was a night….
(most stars would have companions)
…that we might never realise there was more to the
universe than our small cluster of stars.
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