Stars, Galaxies, and the Universe

Metal in Astronomy

In astronomy, "metal" refers to any element other than hydrogen or helium. These are the only elements that occur naturally without the nuclear fusion activity of stars.

Galactic Disk vs. Galactic Halo

• Galactic Disk:
  • Highly flattened
  • Contains both old and young stars
  • Gas and dust are present
  • Ongoing star formation
  • Gas and dust have circular orbits in the galactic plane
  • Spiral-like structure
• Galactic Halo:
  • Roughly spherical
  • Contains only old stars
  • No gas and no dust
  • No ongoing star formation
  • Stars have random orbits in three dimensions
  • No obvious structure

Henrietta Leavitt's Discovery

Henrietta Leavitt discovered the relationship between the luminosity and the period of Cepheid variable stars. She was examining photographic plates in order to measure and catalog the brightness of stars. Though she received little recognition in her lifetime, her discovery first allowed astronomers to measure the distance between the Earth and faraway galaxies.

She explained her discovery: "A straight line can readily be drawn among each of the two series of points corresponding to maxima and minima, thus showing that there is a simple relation between the brightness of the variables and their periods."^[1]

After Leavitt's death, Edwin Hubble used the luminosity-period relation for Cepheids, together with spectral shifts first measured by fellow astronomer Vesto Slipher at Lowell Observatory, to determine that the universe is expanding.

Virgo Cluster

The Virgo Cluster (VC) is a cluster of galaxies whose center is 53.8 ± 0.3 Mly (16.5 ± 0.1 Mpc) away in the constellation Virgo. Comprising approximately 1300 (and possibly up to 2000) member galaxies, the cluster forms the heart of the larger Virgo Supercluster, of which the Local Group (LG) is an outlying member.

Harlow Shapley

Harlow Shapley used RR Lyrae stars to correctly estimate the size of the Milky Way Galaxy and the Sun's position within it by using parallax.^[3] In 1953, he proposed his "liquid water belt" theory, now known as the concept of a habitable zone.^[4]

Globular Clusters

A globular cluster is a spherical collection of stars that orbits a galactic core as a satellite. Globular clusters are very tightly bound by gravity, which gives them their spherical shapes and relatively high stellar densities toward their centers.

Our Sun and its Location

The Solar System is located within the disk, about 27,000 light-years from the Galactic Center, on the inner edge of one of the spiral-shaped concentrations of gas and dust called the Orion Arm.