Our Place Our Place in in the the Cosmos Cosmos Way from within - - PDF document

Our Our Place Place in in the the Cosmos Cosmos

Lecture 16 The Milky Way Galaxy

The Milky Way

• The night sky is filled with a single galaxy -
• ur home, the Milky Way
• Unlike all other galaxies, we view the Milky

Way from within

• We see individual stars and nebulae and also a

band of milky light from billions of unresolved stars [gala = Greek for milk]

• Obscuring clouds of dust appear as dark bands

against the diffuse light

• The Milky Way looks similar to barred spiral

galaxies seen edge-on

Milky Way as seen from Earth The barred spiral galaxy NGC 891

The Milky Way is a Barred Spiral Galaxy Observing the Milky Way

• Although we have a close-up view of the

Milky Way, our view is obscured by dust that lies in the plane of the Galaxy

• This was not known to William Herschel

around the turn of the 19th century, whose model for the Milky Way was based simply on counting the number and brightness of stars in different directions

Herschel’s Model of the Milky Way (c1800) Measuring the Milky Way

• Our primary distance measure - trigonometric

parallax - works only to distances of a few hundred light-years

• A key distance indicator within the Milky Way

are globular clusters

• These are large spheroidal groups of stars

held together by gravity, much like very small elliptical galaxies

• The 150 or so catalogued globular clusters

have luminosities from 400 L to 1 million L

• They typically contain 500,000 stars within a

radius of 15 light-years

Globular Cluster M80 Globular Cluster Distribution

• About 3/4 of globular clusters occupy a

large volume surrounding the disk and bulge known as the halo of the Milky Way

• Three advantages of studying globular

clusters

1. They are very luminous and so can be seen to large distances

• 2. Those outside the disk are relatively unobscured

by dust

• 3. All stars within them are at approximately the

same distance from us and are of approximately the same age and chemical composition

Distances and Ages of Globular Clusters

• If the globular clusters contain stars of

known luminosity L and apparent brightness b, then neglecting dust obscuration, the distance to the cluster is given by

• Stars of known luminosity are referred to as

standard candles

• Examples include the luminosities of the

lowest-mass stars to have left the main sequence - which also provides an age estimate The globular cluster M92 The colour of the main sequence turnoff on the H-R diagram corresponds to a mass

• f 0.8 M and an age
• f about 13 billion

years

Size of the Galaxy

• Harlow Shapley used RR Lyrae variable stars

(whose luminosity is proportional to period of variability) to estimate distances to globular clusters

• Knowing distances to the clusters and their

position on the sky, Shapley could make a 3d map of the distribution of globular clusters

• He found that they occupied a roughly

spherical region of space with a radius of about 300,000 light-years

Stereoscopic views of the distribution of globular clusters in the Milky Way Colours denote average colour

• f stars in each

cluster Sun’s location shown by a green cross

Distribution of Globular Clusters

• Globular clusters move under the gravitational

influence of the Galaxy

• Symmetry requires that the centre of the

distribution of globular clusters coincides with the centre of mass of the Galaxy

• By determining the distance to the centre of

the distribution, Shapley found our distance from the Galactic centre

• Modern determinations show that the Sun is

about 27,000 light-years from the centre - about halfway toward the edge of the disk

Rotation of the Galaxy

• Rotation of the Galaxy is best determined by
• bserving radio waves to which dust clouds

are transparent

• The 21-cm line of hydrogen is particularly

useful

• Doppler shift of measured radiation gives the

radial velocity of the emitting gas cloud

• Observations show that the galaxy is rotating
• Sun is moving on a near-circular orbit at

about 220 km/s

In this direction gas clouds move away from us red-shift In this direction gas clouds move towards us blue-shift

Map of the Milky Way

Milky Way appears to be a typical SBbc giant barred spiral

NGC6744 (SBbc) Milky Way probably looks like this…

Milky Way Rotation Curve

Like nearly all spiral galaxies, the rotation speed of the Milky Way is almost independent of radius to large distances Milky Way is mostly dark matter

Dark Matter in the Milky Way

• Applying Newton’s laws to the observed

rotation curve of the Galaxy yields a total mass of about 6 x 1011 M

• Amount of luminous material would suggest a

much lower mass, hence Milky way is mostly dark matter

• Inner parts are dominated by visible matter,
• uter parts, to at least 150,000 light years,

(the “halo”) are dominated by dark matter

What is the Halo Dark Matter?

• One possibility is dark astronomical-sized objects such

as “Jupiters”, cooled white dwarfs or neutron stars, and black holes

• These are collectively known as Massive Astrophysical

Compact Halo Objects (MACHOs)

• Although dark, their presence may be detected by

their deflection of light, as predicted by Einstein’s General Theory of Relativity

• If they pass directly in front of a distant star, the

star’s light will be deflected and amplified - an effect known as gravitational lensing

The MACHO Experiment The MACHO Experiment

• The MACHO experiment monitored tens of

millions of stars over several years in the Large and Small Magellanic Clouds, two small satellite galaxies of the Milky Way

• Around twelve gravitational lensing events

were found, but not nearly enough to explain all of the dark matter in our halo

• Thus while some MACHOs exist, it is unlikely

that our dark matter halo is primarily composed of MACHOs

A Supermassive Black Hole at the Galaxy’s Centre

• We saw in the last lecture that most galaxies

have a supermassive black hole (SMBH) at their centres

• The Milky Way is no exception
• Observations of the orbits of stars very close

to the centre of the Galaxy indicate the presence of a SMBH of mass around four million M

• Our Galaxy was probably once “active” - all

that now remains of the AGN is a quiescent SMBH

Near-infrared image of the centre of the Galaxy (blue=hot, red=cool) Centred on Sgr A* 1 ly = 8”

A Closer View in K-band 2.1µ Results

• Position of the star “S2” measured over a 10-

year period 1992-2002 gives almost a complete orbit: an ellipse with Sgr A* at one focus

• Orbital period = 15.2 years (cf 200 Myr for

Sun)

• Pericentric distance (closest approach) 17 light

hours - only 3 times Sun-Pluto distance (cf ~25,000 light years for Sun)

Mass within orbit

• Speed at closest approach ~5000 km/s (200

times faster than Sun’s orbital speed around Galaxy)

• For star in circular orbit of radius r about

point mass M, Kepler’s laws give velocity v as v2 = GM/r

• Allowing for eccentric orbit (e=0.87) gives a

mass of 3.7 x 106 Solar masses within a radius of 17 light hours of Sgr A*

Summary

• We believe the Milky Way is a fairly typical

barred spiral galaxy

• We live about half-way out in the disk, about

27,000 light-years from the centre

• The mass of the Galaxy is dominated by dark

matter

• A supermassive black hole of around four

million M lies at the Galactic centre