Astronomy Unit 5: Hubble's Classification

Study Notes

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.

Description

Explore Hubble's Classification of galaxies through this exciting quiz. Understand the different types: elliptical, spiral, barred spiral, and irregular galaxies as well as their representations in the Hubble sequence. Test your knowledge and see how well you grasp this cosmic classification system.