Astronomy Unit 5: Hubble's Classification

Hubble's Classification

• Hubble's galaxy classification scheme categorizes galaxies into four types: elliptical, spiral (and barred spiral), and irregular.
• The "tuning fork" diagram visualizes this classification, showing the relationship between types.
• A d-type spiral galaxy has loosely wound arms and a smaller core.
• The Milky Way is categorized as a barred spiral galaxy (SBb).

Major Types of Galaxies

• Galaxies fall into three major categories: spiral, elliptical, and irregular.

Spiral Galaxies

• Spiral galaxies, including the Milky Way, appear as flat white disks with yellowish bulges at their centers.
• They contain cool gas, dust, and ionized gas, featuring prominent spiral arms.
• Stars in spiral galaxies are divided into disk components and halo components.
• They represent 20% of all galaxies and have significant amounts of dust and gas.

Irregular Galaxies

• Irregular galaxies lack a defined shape, appearing chaotic and disorganized.
• Their structure often results from near-collisions or actual collisions with other galaxies.
• Examples include the Magellanic Clouds, which are satellite galaxies of the Milky Way.

Elliptical Galaxies

• Elliptical galaxies are redder and rounder, resembling elongated football shapes.
• They have little cool gas and dust compared to spiral galaxies but may contain hot, ionized gas.
• Approximately 80% of all galaxies are elliptical, with many being dwarf galaxies.

Disk Component

• The disk component comprises stars and gas clouds that orbit the galactic center in nearly circular paths.
• Galaxy disks contain various forms of gas, differing in proportions among galaxies.

Halo Component

• The halo component includes the rounded or elliptical-shaped halo and central bulge.
• It contains minimal cool gas and dust, with stars having diverse orbital inclinations.
• The disk of a galaxy is situated within the halo.

Barred Spiral Galaxies

• Some spiral galaxies feature a central bar of stars, known as barred spirals.
• Astronomers believe the Milky Way is a barred spiral due to the elongation of its bulge.

Differences Between Elliptical and Spiral Galaxies

• Elliptical galaxies have only a halo component, lacking a significant disk component.
• They resemble the bulge and halo of spiral galaxies without the disk.
• Giant elliptical galaxies are rare but very massive, while dwarf elliptical galaxies are common.
• Elliptical galaxies are typically red or yellow due to low star formation activity and lack of hot, young stars.

Galaxies Grouping

• Spiral galaxies often form small groups, whereas elliptical galaxies predominantly exist in large clusters.

Standard Candles

• Standard candles are astronomical objects with known luminosity, allowing distance measurement through brightness comparison.