Galaxies are faint

Copyright: 20th Century Fox

Clustering

• Quarks and electrons are fundamental (?)
• Quarks make up protons and neutrons
• Protons, neutrons and electrons make up atoms
• Atoms make up molecules
• Molecules make up solids, liquids and gases
• Gases make up stars
• Stars make up galaxies
• Galaxies make up galaxy clusters
• Galaxy clusters make up super-clusters

Our galaxy - the Milky Way

• Several hundred billion stars.
• Mass is about a thousand billion solar masses.
• About 100,000 light years in diameter.
• Oldest known star is 13.6 billion years old.
• Appears as a faint, fuzzy band encircling the Earth.

The Milky Way - from inside

Source: Steve Jurvetson CC BY 2.0

The Milky Way - from outside

Source: NASA/JPL-Caltech/ESO/R. Hurt Public Domain

The Milky Way - schematic

Source: RJHall CC-BY SA 3.0

Metallicity in our galaxy

Astronomers refer to the fraction of all elements other than H and He as metallicity.

• stars in the disk have higher metallicity and are younger
• stars in globular clusters have lower metallicity and are older
• many stars in the galactic bulge are older with low metallicity

Observing hydrogen

• Our galaxy contains many gas clouds mainly composed of hydrogen.
• Hydrogen emits radio waves at a wavelength of 21 cm.
• We can use this line to find hydrogen clouds and map out our galaxy.
• And, we can measure the speed at which they move towards or away from us (the Doppler effect, see earlier).

The Milky Way - observations

Source: Rursus CC-BY SA 3.0

The Milky Way - rotation

Red - theory; Blue - observed; Yellow - the Sun

Source: Brews ohare CC-BY SA 3.0

Dark matter

• The graph shows the speed of rotation at different distances from the centre of the galaxy
• 1 kpc = 1000 parsecs = 3260 light years.
• The red curve is the theory based on mass we can see in the galaxy.
• The blue curve are the observations (with error bars).
• The fact that they are different means there is mass we cannot see - dark matter.

Small and large Magellanic clouds

Source: ESO/S. Brunier CC-BY SA 3.0

Satellite galaxies?

• The LMC and SMC are dwarf galaxies passing the Milky Way.
• It was previously thought they orbited the Milky Way.
• The LMC is 160,000 light years away.
• The SMC is 200,000 light years away.