Download Lecture 10: The Milky Way

X.
The Milky Way
h"p://sgoodwin.staff.shef.ac.uk/phy111.html 0. Our home galaxy
Most stars visible from the Earth are concentrated in the
band of the Milky Way.
This is our view from the inside of our Milky Way galaxy (or
‘the Galaxy’) which contains some 100-400 billion stars.
1. A disc galaxy
That most stars lie in a band around the sky from the Earth
suggests we live in a disc galaxy. But how big is it, and
where are we in it?
1. The disc
Most stars locally are concentrated in the disc, but are fairly
smoothly distributed within the disc.
The disc is around 1kpc thick, but can be divided into two
populations:
The thin disc is concentrated near the mid-plane of the
disc and is mostly made of young (<8Gyr), metal-rich stars.
The thick disc is more diffuse and extends out to 1kpc and
is made of older, more metal-poor stars.
Typically the stellar density in the disc is about 0.1 star pc-3
(typical separations between stars of about 1pc).
1. Metallicity
The metal (ie. everything apart from H and He) content of
stars is usually measured as a log relative to the Sun.
[Fe/H] is the iron-to-hydrogen ratio of a star relative to the
Sun.
[Fe/H]=-1 means there is 0.1 of the Sun’s Fe/H.
[O/H]=+0.3 means there is 2x the Sun’s O/H.
[Fe/O]=0 means there is the same (x1) Fe/O as the Sun.
Typically [Fe/H] is used as it is easy to measure from
optical spectra (Fe has lots of lines in the visible).
1. Distances
We only have parallax distance measurements out to about
100pc (Gaia will take us much further in the next few
years).
This gives us the absolute luminosities of low-mass stars,
and using binary systems we can calibrate our models to
true masses and radii (see earlier).
The trouble is that within 100pc we have no massive stars
and only 4 giants – how do we calibrate these?
To get distances to objects further away we use a series of
steps on the ‘distance ladder’
2. Star clusters
A few per cent of stars are found in star clusters: ~1pc
radius gravitationally bound groups.
Spectroscopy tells us that all the stars in a star cluster have
almost exactly the same chemical composition – which
suggests they were born together at the same time.
2. Star clusters
The HR diagrams of star clusters look very ‘clean’
Because all of the stars are the
same age all stars above a
particular mass have ‘turned off’
the MS onto the giant branches.
By using our knowledge of local
MS stars we can find the
distance to the cluster, and also
find the mass of the turnoff.
2. Star clusters
We understand MS lifetimes pretty well, so knowing the
turnoff mass tells us the age of the cluster. We now have
the age and distance of all the stars in the cluster.
The higher the turnoff mass – the younger the cluster.
This is the Pleiades – the
turnoff is at about 90L
From lecture 5 this is a mass
of about 6M, and from
lecture 6 this gives a MS
lifetime of ~100Myr.
2. Variable stars
Some star clusters contain types of unstable AGB stars that
have regular pulsations (they lie on the ‘instability strip’).
The most important of these variables are Cepheid
variables – with luminosities of up to 104 L they can be
seen in distant galaxies (Polaris is a nearby Cepheid
variable).
Their period is related to their luminosity – so if you observe
one pulsating, you can calculate its absolute luminosity and
so its distance.
These become very important later, but are calibrated
locally when found in clusters.
2. Open clusters
When we look at the region that contains most of the stars
in the Milky Way (the disc) we find that clusters tend to be
a)  fairly small (103-104 M),
b)  generally young (<1 Gyr),
c)  have similar levels of metals to the Sun.
We call these clusters open clusters.
2. Globular clusters
When we look away from the main Milky Way we find that
clusters tend to be
a)  Massive (104-106 M),
b)  Very old (~12 Gyr),
c)  Have very few heavy elements.
We call these clusters globular clusters.
3. The size of the Milky Way
If we plot the distribution of globular clusters we find they lie
in a sphere whose centre is about 8kpc from the Sun. Most
globular clusters lie within about 10kpc of the centre, but
some lie as far as 100kpc away. This spherical distribution
of very old stars and star clusters is known as the halo.
3. The bulge
When we look at the centre of the Milky Way as found by
the globular clusters we find the spherical bulge (only
visible from the south). This is about 4kpc in radius, and
mostly old, metal-poor stars, but does have some gas and
young stars as well.
4. The structure of the MW
We live in a galaxy that has three major components of
different ages and metallicities.
Disc (thin+thick) – about 25kpc in radius, only about 1kpc
thick. Most of the stars are young (0-8 Gyr), and have
about the same metal content as the Sun. Total stellar
mass of about 6x1010M.
Bulge – a mostly old (10 Gyr), metal-poor population in a
~4kpc sphere, but does contain some young stars. Total
stellar mass of about 1010M.
Halo – a very old, very metal-poor spherical population out
to >100 kpc. Total stellar mass of about 0.3x1010M.
4. The structure of the MW
Summary
The Milky Way is a spiral galaxy. It has disc, bulge and
halo components with different masses, ages, and
metallicities.
We live around 8kpc from the Galactic centre near the midplane of the disc.
Key points
To describe the structure and make-up of the Milky Way
both in words and diagramatically.
To describe how we determine the size of the Milky Way
and our distance from the centre.
Example short questions
A cluster is observed to be 100 Myr old. In what component of the Galaxy would you
expect it to be located and why?
A cluster is observed to be 12 Gyr old. In what component of the Galaxy would you
expect it to be located and why?
Sketch the structure of the Milky Way, naming and indicating the main stellar
components.
A star is observed with a metallicity of 10-3 of that of the Sun. In what component of the
Galaxy would you expect it to be located?
Would you expect a core collapse supernova to occur in the halo of the Milky Way?
Briefly justify your answer.