Edwin Hubble and the Classification of Galaxies

Edwin Hubble

Astronomer who proved existence of other galaxies.

Cepheid variables

Stars used to measure distances in galaxies.

Andromeda Nebula

First galaxy identified as separate from Milky Way.

Elliptical galaxies

Contain old Pop II stars, few heavy elements.

Dwarf elliptical galaxies

Small, common type of galaxy, hard to observe.

Milky Way

Barred spiral galaxy, two-thirds of spiral types.

Irregular galaxies

Smaller galaxies with many newly formed stars.

Mass-to-Light ratio

Indicates star sizes; low means large stars.

Distance measurement

Cepheid variables effective up to 100 million light years.

Tully-Fisher technique

Measures galaxy rotation using hydrogen emission.

Type Ia supernovas

Standard candles with consistent luminosity.

Hubble's Law

Distance measurement using redshift for far galaxies.

Barred spiral

Spiral galaxy with a central bar of stars.

Galaxy classification

Hubble classified galaxies primarily by shape.

Old stars

Predominant in elliptical galaxies, less blue.

Luminosity

Brightness of a galaxy, linked to rotation speed.

Galactic clusters

Groups of galaxies, can contain large elliptical galaxies.

Gas and dust

Up to 50% of material in irregular galaxies.

Galactic mergers

Can change the type or shape of galaxies.

H-R diagram

Graph showing relationship between stars' luminosity and temperature.

Mass estimation

Galaxy mass determined by outer star or gas speed.

Andromeda Galaxy mass

Estimated to be somewhat larger than Milky Way.

Galaxy visibility

Dwarf elliptical galaxies are often difficult to see.

Star formation

Irregular galaxies have high rates of new star formation.

Galactic colors

Elliptical galaxies appear yellow and red due to older stars.

Spiral galaxy characteristics

Flattened disk shape with gas and dust.

Supernova explosions

Used as standard bulbs for distance measurement.

Galactic rotation

Speed relates to luminosity in Tully-Fisher technique.

Galaxies

Massive systems containing stars, gas, and dust.

Dark Matter

Invisible matter increasing a galaxy's mass-to-light ratio.

Mass-to-Light Ratio

Comparison of a galaxy's mass to its luminosity.

Hubble Space Telescope

Instrument measuring distances to distant galaxies.

Cepheid Variables

Stars used to measure distances via period-luminosity relation.

Redshift

Shift in light spectrum indicating an object's velocity.

Type Ia Supernovae

Standard candles for measuring astronomical distances.

Tully-Fisher Relation

Correlation between spiral galaxy luminosity and rotation speed.

Rotational Velocity

Speed at which a galaxy rotates, influencing luminosity.

Doppler Shift

Change in frequency of light due to motion.

Milton Humason

Astronomer who aided Hubble in universe expansion studies.

Vesto Slipher

First to show spiral nebulae had large Doppler shifts.

Spectroscope

Instrument analyzing light to determine elemental composition.

Hydrogen

Most common element found in distant galaxies.

Blue Shift

Indicates an object is moving toward the observer.

Expansion of Universe

Observation that galaxies are moving away from each other.

Accretion Disks

Disks of gas and dust around massive celestial objects.

Main Sequence

Stage in stellar evolution where stars fuse hydrogen.

H-R Diagram

Graph plotting stars' brightness against their temperature.

Local Group

Galaxy cluster including the Milky Way and Andromeda.

Gamma-ray Emission

High-energy radiation from cosmic events, like supernovae.

Spectral Lines

Patterns in light spectra indicating elemental presence.

Accretion

Process of matter falling into a gravitational well.

Supernova Remnants

Leftover material from a star's explosive death.

Galactic Bulge

Dense region of stars at a galaxy's center.

Open Clusters

Groups of stars loosely bound by gravity.