16 Galaxies

1. Other Galaxies

Almost every object visible is a galaxy

(a) A collection of many galaxies, each consisting of hundreds of billions of stars. Called the Coma Cluster, this group of galaxies lies more than 100 million pc from Earth.

(b) A recent Hubble Space Telescope image of part of the cluster.

Coma Cluster. The blue spiked object at top right is a nearby star. The rest of objects are all galaxies.

2. Hubble Galaxy Classification

Hubble in 1924 used 2.4 m telescope at Mount Wilson to classify galaxies. Using the photographics plates, he divided all the galaxies into four main groups which is known as Hubble Galaxy Classification: Spirals (S), Barred Spirals (SB), Ellipticals (E), Irregulars (Irr)

Spiral Galaxies

  • They are classified according to the size of their central bulge.

  • As we progress from type Sa to Sb to Sc, the bulges become smaller and the spiral arms tend to become less tightly wound.

  • The components of spiral galaxies are the same as in our own galaxy: disk, core, halo, bulge, and spiral arms.

Sombrero Galaxy The Sombrero Galaxy, a spiral system seen edge-on. Officially cataloged as M104, this galaxy has a dark band composed of interstellar gas and dust. The large size of this galaxy’s central bulge marks it as type Sa, even though its spiral arms cannot be seen from our perspective. The inset shows this galaxy in IR, highlighting its dust content in false-colored pink.

Barred Spiral Galaxies

  • The variation from SBa to SBc is similar to that for the spirals except that now the spiral arms begin at either end of a bar through the galactic center.

  • In frame (c), the bright star is a foreground object in our own Galaxy; the object at top center is another galaxy that is probably interacting with NGC 6872.

Lenticular Galaxies

  • (a) S0 (or lenticular) galaxies contain a disk and a bulge, but no interstellar gas and no spiral arms. They are in many respects intermediate between E7 ellipticals and Sa spirals in their properties.

  • (b) SB0 galaxies are similar to S0 galaxies, except for a bar of stellar material extending beyond the central bulge.

Elliptical Galaxies

  • Ellipticals are classified according to their shape from E0 (almost spherical) to E7 (the most elongated)

  • They lack spiral structure, and neither shows evidence of cool interstellar dust or gas, although each has an extensive X-ray halo of hot gas that extends far beyond the visible portion of the galaxy.

  • Note in (c): M110 is a dwarf elliptical companion to the much larger Andromeda Galaxy.

Irregular Galaxies

  • The Magellanic Clouds are dwarf irregular (Irr I) galaxies, gravitationally bound to our own Milky Way Galaxy. They orbit our Galaxy and accompany it on its trek through the cosmos.

  • Both have distorted, irregular shapes, although some observers claim they can discern a single spiral arm in the Large Cloud.

  • Bottom panel: Some irregular (Irr II) galaxies.

  • (a) The strangely shaped galaxy NGC 1427A is probably plunging headlong into a group of several other galaxies (not shown), causing huge rearrangements of its stars, gas, and dust.

  • (b) The galaxy M82 seems to show an explosive appearance.

3. Distribution of Galaxies in Space

Cepheids extend the distance scale to 25 Mpc. However,

  • Some galaxies have no Cepheids.

  • Most of them are farther away then 25 Mpc

Therefore a new distance measures are needed: Tully–Fisher relation correlates a galaxy’s rotation speed (which can be measured using the Doppler effect) to its luminosity.

Galaxy Rotation

  • A galaxy’s rotation causes some of the radiation it emits to be blueshifted and some to be redshifted (relative to what the emission would be from an unmoving source).

  • From a distance, when the radiation from the galaxy is combined into a single beam and analyzed spectroscopically, the redshifted and blueshifted components combine to produce a broadening of the galaxy’s spectral lines.

  • The amount of broadening is a direct measure of the rotation speed of the galaxy.

Standard Candle Method

  • Type I Supernovae have about the same luminosity, as the process by which they happen doesn’t allow for much variation.

  • They can be used as “standard candles” and which can therefore be used to determine distance using their apparent magnitude.

Local Group

It is made up of nearly 50 galaxies within approximately 1 Mpc of our Milky Way Galaxy.

  • Only a few are spirals; most of the rest are dwarf-elliptical or irregular galaxies, only some of which are shown here.

  • The inset map (top right) shows the Milky Way in relation to some of its satellite galaxies.

  • The photographic insets (top left) show two well-known neighbors of the Andromeda Galaxy (M31): the spiral galaxy M33 and the dwarf elliptical galaxy M32 (also visible in Figure 23.2a, a larger-scale view of the Andromeda system).

Such a group of galaxies, held together by its own gravity, is called a galaxy cluster.

Virgo Cluster

It contains about 3500 galaxies. It is about 17 Mpc away from Earth.

  • Many large spiral and elliptical galaxies can be seen.

  • The inset shows several galaxies surrounding the giant elliptical known as M86.

  • An even bigger elliptical galaxy, M87, noted at the bottom.

Abell 1689 galaxy cluster

It contains huge numbers of galaxies and resides roughly 1 billion parsecs from Earth.

  • Virtually every patch of light in this photograph is a separate galaxy.

  • We also see many galaxies colliding, some tearing matter from one another, others merging into single systems.

4. Hubble's Law

Universal recession

All galaxies (with a couple of nearby exceptions) seem to be moving away from us, with the redshift of their motion correlated with their distance

  • Optical spectra of several galaxies named on the right.

  • Both the extent of the redshift (denoted by the horizontal red arrows) and the distance from the Milky Way Galaxy to each galaxy (numbers in center column) increase from top to bottom.

  • The vertical yellow arrow in each spectrum highlights a particular spectral feature (a pair of dark absorption lines).

  • The horizontal red arrows indicate how this feature shifts to longer wavelengths in spectra of more distant galaxies.

  • The white lines at the top and bottom of each spectrum are laboratory references.

Hubble's Law

Plots of recessional velocity against distance:

  • (a) for nearby galaxies shown above

  • (b) for numerous other galaxies within about 1 billion pc of Earth.

The relationship (slope of the line) is characterized by Hubble’s constant H0:

recessional velocity = H0 × distance

The currently accepted value for Hubble’s constant

H0 = 70 km/s/Mpc

Measuring distances using Hubble’s law actually works better on farther away objects; random motions are overwhelmed by the recessional velocity.

Cosmic Distance Ladder

  • Hubble’s law tops the hierarchy of distance measurement techniques.

  • It is used to find the distances of astronomical objects all the way out to the limits of the observable universe.

5. Active Galactic Nuclei (AGN)

Beyond the class of Normal Galaxies

  • About 20-25% of all galaxies don't fit to Hubble Classification.

  • They are far too luminous.

  • They are different than "Normal Galaxies": Luminosity & Type of Radiation.

  • Excess luminosity is due to Non-stellar Radiation.

  • When a galaxy interact with its neighbor it might trigger an outburst of star formation - called starbust galaxies.

  • Excess luminosity might be also due to activity in and around the galactic center.

Active Galactic Nuclei

  • Luminous Center

  • Star Formation Rings around the center


  • Seyfert Galaxies

  • Radio Galaxies

  • Quasars

The active galaxy NGC 7742. It resembles a fried egg, with a ring of blue star-forming regions surrounding a very bright yellow core that spans about 1 kpc. It combines star formation with intense emission from its central nucleus.

Seyfert Galaxies

  • They resemble normal spiral galaxies

  • But their cores are thousands of times more luminous.

  • (a) The Circinus galaxy with a bright compact core.

  • It lies some 4 Mpc away.

  • It is one of the closest active galaxies.

  • (b) The rapid variations in the luminosity indicate that the core must be extremely compact.

  • The variation is Radio wavelength. However, optical and X-ray luminosities vary as well.

Radio Galaxies

  • (a) Centaurus A Radio Lobes

  • (b) It has a giant radio-emitting lobes extending a million parsecs or more beyond the central galaxy.

  • This entire object is thought to be the result of a collision between two galaxies that took place about 500 million years ago.

  • The lobes are shown here in pastel false colors, with decreasing intensity from red to yellow to green to blue.

  • The inset (at right) shows an X-ray image of one of the lobes up close, proving that at least the jets in the inner parts of the lobes emit X rays.

Core-Dominated Radio Galaxy

  • (Left) M86. The radio emission comes from a bright central nucleus, which is surrounded by an extended, less-intense radio halo.

  • (Right) A central energy source produces high-speed jets of matter that interact with intergalactic gas to form radio lobes.

  • The system may appear to us as either radio lobes or a core-dominated radio galaxy, depending on our location with respect to the jets and lobes.

The giant elliptical galaxy M87 (also called Virgo A) is displayed here in a series of zooms.

  • (a) A long optical exposure of the galaxy’s halo and embedded central region.

  • (b) A short optical exposure of its core and an intriguing jet of matter, on a somewhat smaller scale.

  • (c) An IR image of M87’s jet, examined more closely compared with (b).

  • The bright point at left marks the bright nucleus of the galaxy; the bright blob near the center of the image corresponds to the bright “knot” visible in the jet in (b).


  • They are named as Quasi Stellar Objects; they are starlike in apparance but have very unusual spectral lines.

  • (a) The bright quasar 3C 273 displays a luminous jet of matter, but the main body of the quasar is starlike in appearance.

  • (b) The jet extends for about 30 kpc and can be seen better in this high-resolution image.

  • (Right - Top) Optical spectrum of the distant quasar 3C 273.

  • Notice both the redshift and the widths of the three hydrogen spectral lines marked as Hβ, Hγ, and Hδ.

  • The redshift indicates the quasar’s enormous distance. The width of the lines implies rapid internal motion within the quasar.

  • (Right - Bottom) Typical Quasar

  • 1) Quasars are the most luminous objects in the universe.

  • 2) Quasars's much greater distance makes it appear fainter than the stars - but intrinsically it is much, much birghter.

6. Cental Engine of an AGN

Properties of AGNs

  • high luminosity

  • nonstellar energy emission

  • variable energy output, indicating small nucleus

  • jets and other signs of explosive activity

  • broad emission lines, indicating rapid rotation


  • The energy source in AGN holds that these objects are powered by material accreting onto a supermassive black hole.

  • As matter spirals toward the hole, it heats up, producing large amounts of energy.

  • At the same time, high-speed jets of gas may be ejected perpendicular to the accretion disk, forming the jets and lobes seen in many active objects.

  • Magnetic fields generated in the disk are carried by the jets out to the radio lobes, where they play a crucial role in producing the observed radiation.

  • The accretion disk is whole clouds of interstellar gas and dust; they may radiate away as much as 10–20% of their mass before disappearing.

Giant Elliptical Galaxy

  • (a) A combined optical/radio image of the giant elliptical galaxy NGC 4261, in the Virgo Cluster, shows a white visible galaxy at center, from which blue-orange (false-color) radio lobes extend for about 60 kpc.

  • (b) A close-up photograph of the galaxy’s nucleus reveals a 100-pc-diameter disk surrounding a bright hub thought to harbor a black hole.

M87 Disk

  • Recent images and spectra of M87 support the idea of a rapidly whirling accretion disk at the galaxy’s heart.

  • (a) An image of the central region of M87 shows the galaxy’s bright nucleus and jet.

  • (b) A magnified view of the nucleus suggests a spiral swarm of stars, gas, and dust.

  • (c) Spectral-line features observed on opposite sides of the nucleus show opposite Doppler shifts.

  • The implication is that an accretion disk spins perpendicular to the jet and that at its center is a black hole having some 3 billion times the mass of the Sun.

Dusty Donut

  • The accretion disk surrounding a massive black hole consists of hot gas at many different temperatures (hottest nearest the center).

  • When viewed from above or below, the disk radiates a broad spectrum of electromagnetic energy extending into the X-ray band.

  • However, the dusty infalling gas that ultimately powers the system is thought to form a rather fat, donut-shaped region outside the accretion disk (shown here in dull red), which effectively absorbs much of the high-energy radiation reaching it, re-emitting it mainly in the form of cooler, infrared radiation.

  • Thus, when the accretion disk is viewed from the side, strong infrared emission is observed.

  • For systems with jets (as shown here), the appearance of the jet, radiating mostly radio waves and X rays, also depends on the viewing angle.

Nonthermal Radiation

  • (a) Charged particles, especially fast-moving electrons (red), emit synchrotron radiation (blue) while spiraling in a magnetic field (black).

  • It also occurs on smaller scales as well:

    • in Earth’s Van Allen belts

    • charged matter arches above sunspots

    • in the vicinity of neutron stars

    • at the center of our own Galaxy

  • (b) Variation of the intensity of thermal and synchrotron (nonthermal) radiation with frequency.

  • Thermal radiation, described by a blackbody curve, peaks at some frequency that depends on the temperature of the source.

  • Nonthermal synchrotron radiation, by contrast, is more intense at low frequencies and is independent of the temperature of the emitting object.