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Galaxies HST 2006. David Ernest & Joao Pedro Saraiva. What is a Galaxy?. The stars don’t lie alone, lost in the middle of the universe. They are grouped together, by billions, in structures that are called galaxies . These galaxies are extremely numerous.

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galaxies hst 2006
GalaxiesHST 2006

David Ernest & Joao Pedro Saraiva

what is a galaxy
What is a Galaxy?

The stars don’t lie alone, lost in the middle of the universe. They are grouped together, by billions, in structures that are called galaxies.

These galaxies are extremely numerous.

Their number is estimated to be at least

150 billion in the visible universe.



The nearest big spiral galaxy to the Milky Way

  • Andromeda's visible mass: 300 to 400 billion Solar-masses
  • Andromeda's disk have a diameter of over 200,000 ly

(assuming a distance of 2.5 million ly to the Milky Way)


Galaxies can take a lot of shapes, very smooth or very irregular, with two, four or six arms. However far you look in the universe, you will find galaxies

The challenge astronomers face is interpreting these two-dimensional pictures of a four-dimensional universe

After all, everything along the line of sight—near and far, young and old—is projected onto the same plane of the sky

why is the night sky dark
Why is the Night Sky Dark?
  • Olber's Paradox:
    • Suppose that the universe is (1) static, (2) infinite, (3) eternal and (4) uniformly filled with stars;
    • If we look in any direction, our line of sight must eventually run into a star;
    • Therefore, the night sky should be as bright as the average star and certainly should not be dark!

Maybe there's matter out there that's blocking

the energy from reaching us?

Possibly, but in the end, that matter would heat up, and then it would radiate out energy, as per thermal radiation of a black body. The heat would reach us anyway.


The fact that the Universe has a finite age is the principal explanation of Olber's Paradox:

We live inside a spherical shell of "Observable Universe" which has radius equal to the lifetime of the Universe. Objects more than about 14 billion years old are too far away for their light ever to reach us.


According to the classification scheme developed by Hubble in the 1920s, galaxies may be broadly divided into three major types:

Elliptical, Spiral and Irregular

(This is not an evolution sequence)

how the milky way formed
How the Milky Way Formed?
  • The Milky Way system is a spiral galaxy consisting of over 400 billion stars, plus gas and dust arranged into three general components:
    • The halo: a roughly spherical distribution which contains the oldest stars in the Galaxy including Globular Clusters;
    • The nuclear bulge and Galactic Center: that harbours a compact object of very large mass, strongly suspected to be a supermassive black hole;
    • The disk of the Galaxy: is a flattened, rotating system which contains the Sun and other intermediate-to-young stars. The sun sits about 2/3 of the way from the center to the edge of the disk.

The Milky Way is a barred spiral galaxy made of four main arms curving around its centre.

The sun revolves around the center of the galaxy about once every 250 million years.


The Milky Way in Seven Wavelength Regions

This multiple-strip image shows the plane of the Milky Way galaxy as viewed from Earth in each of several different wavelength bands.

Source: NASA Astrophysics Data Facility, The Multiwavelength Milky Way


The Milky Way forms part of a group of galaxies: The Local Group

  • This group comprises over 30 galaxies, with its gravitational center
  • located somewhere between the Milky Way and the Andromeda Galaxy.
  • The galaxies of the Local Group cover a 10 million light year diameter
  • and belongs to the Virgo Supercluster.
the milky way s satellite system
The Milky Way's satellite system
  • Large Magellanic Cloud
  • Small Magellanic Cloud
  • Sagittarius Elliptical Galaxy
  • Canis Major Dwarf
  • Ursa Minor Dwarf
  • Draco Dwarf
  • Carina Dwarf
  • Sextans Dwarf
  • Sextans Dwarf
  • Sculptor Dwarf
  • Fornax Dwarf
  • Leo I
  • Leo II
  • Tucana Dwarf
  • Ursa Major Dwarf

How did galaxies form?

  • The most accepted view is that all the structure we observe today was formed as a consequence of the growth of primordial fluctuations by gravitational instability.
  • Recent data suggests that the first galaxies formed as early as 600 million years after the BigBang.

How do galaxies change over time?

One of the most important ways that galaxies evolve is through interaction with other galaxies:


The Cartwheel Galaxy

The striking ring-like feature is a direct result of a smaller intruder galaxy that careened through the core of the host galaxy. Like a rock tossed into a lake, the collision sent a ripple of energy into space, blowing gas and dust in front of it.

doppler shift of the spectra
Doppler-Shift of the Spectra

Light from moving objects will appear to have different wavelengths depending on the relative motion of the source and the observer. Observers looking at an object that is moving away from them see light that has a longer wavelength than it had when it was emitted (a redshift), while observers looking at an approaching source see light that is shifted to shorter wavelength (a blueshift).

redshift of different objects
Redshift of different objects

The spectra below show the dark absorption lines. These lines can be used to identify the chemical elements in distant stars, but because of their Doppler-shift they also tell us the radial velocity of the star. Most objects show a redshift in their spectra, which means they are receding from us.

Distant galaxy

Medium distance galaxy

Nearby Galaxy

Star of our galaxy

hubble expansion of universe
Hubble Expansion of Universe

During 1920‘s and 30‘s Edwin Hubble was Measuring the distances of stars and galaxies and comparing them to the redshift of their spectra. He discovered that the radial speed of the galaxies increased in proportion to their distance. A plot of later data is shown in the picture below.

hubble equation
Hubble equation

Hubble constant:

According to Hubble‘s equation the universe is expanding. So it can be calculated how long it took from a single point to become ist today‘s size:

Age of the universe:

rotation of galaxies
Rotation of Galaxies

Can we see the galaxies turn?

What are rotation curves?

Why are rotation curves of galaxies so strange???

can we see the galaxies turn
Can we see the galaxies turn?

A galaxy needs typically 200 Million years to turn once around its axis. By what angle does it turn in one year and can this angle be measured with a telescope?

In 1 year it turns only by 0.006 arc-seconds!

This angle is to small to be measured directly…


Measurement of Doppler-shifts across the galaxy!

rotation curve for a large central mass planet like rotation
Rotation curve for a large central mass: „planet-like rotation“

In most galaxies the largest part of their visible mass is concentrated near the centre. In Analogy to the Solar system, we would expect the stars near the galactic centre to move much faster than those near the edge of the galaxy.

Corresponding rotation curve



Rotation curve from measured Doppler-shifts does not correspond to the distribution of visible mass within the galaxy!

Galaxy NGC 2403



To explain the measured rotation curve, there must be additional invisible mass present within the galactic disk and in a region called halo. This „dark matter“ can make up to 90% of the gravitational mass. Only 10% are visible!



gravitational lensing
Gravitational lensing

The gravity from the massive object in front bends the light which is coming from the objects behind. The path of the light from the source is curved. As a result, the object in the background appears distorted, and its apparent position is changed. The observer may even see multiple images of a single source.


… and dark matter?

From the lensing effect the mass map of the foreground can be reconstructed. Note the large smooth distribution of apparenly invisible matter in the picture below. Visible mass is only about 1% of the total gravitational mass.

what is dark energy
What is „Dark Energy“?

By measuring the expansion rate of the universe in past, i.e. at very large distances we find that the expansion of the universe is accelerating. But the gravity would rather decelerate the expansion!

Scientists try to explain this by the presence of so called “dark energy” or “vacuum energy”.