Halo

The Milky Way halo allows us to peer past the gas and dust that obscures the disk and Galactic center. Learn more about the halo here!

Structure

Spherical Shape

The Milky Way's halo is an expansive, roughly spherical region enveloping the central bulge and the disk. Its morphology is more diffuse and extends far beyond the visible disk, reaching into the outer reaches of the galaxy. Compared to the flattened disk, the halo lacks the pronounced structure and density of stars and interstellar material.

Stellar Population

Stars within the halo are primarily older and belong to Population II, characterized by low metallicities. These stars formed early in the galaxy's history, before significant enrichment by heavier elements occurred. The scarcity of young stars suggests a lower rate of ongoing star formation within the halo compared to the actively star-forming regions in the disk.

Globular Clusters

The halo hosts numerous globular clusters, densely packed collections of ancient stars. These clusters, containing some of the oldest stars in the galaxy, orbit the galactic center on highly elliptical paths within the halo. They are distributed in a roughly spherical halo shape around the center of the Milky Way.

Dynamics

Motion and Orbits

Stars and objects in the halo exhibit different orbital characteristics compared to those in the disk. Halo stars often follow more elongated, elliptical orbits with varying inclinations, contributing to a more random and less ordered motion compared to the predominantly circular orbits found in the disk.

Dark Matter Halo

The halo is thought to contain a significant amount of dark matter, an invisible form of matter detected through its gravitational effects on visible matter. The presence of dark matter in the halo, inferred from observations of stellar motions and galaxy dynamics, contributes substantially to the total mass of the galaxy.

Observations

Challenges

Observing the halo presents difficulties due to its low-density nature and the limited number of bright stars compared to the densely populated regions within the disk. Additionally, dust and gas in the disk can obstruct views of the halo, making observations challenging in certain wavelengths of light.

Techniques

Astronomers employ various methods to study the halo, including surveys targeting specific types of stars known as standard candles, such as RR Lyrae variables and cepheid variables. These stars have well-defined luminosities, making them useful for estimating distances to objects in the halo. Gravitational lensing studies, which analyze the bending of light from background sources due to the galaxy's gravitational field, also provide insights into the halo's mass distribution and structure.

Galactic Evolution

The halo serves as a repository of ancient stars and structures that offer crucial clues about the early stages of the Milky Way's formation and evolution. Its Population II stars, globular clusters, and interaction with dwarf galaxies provide valuable information about the conditions and processes prevailing in the early universe. Studying the halo's properties and contents is pivotal in understanding the assembly history, dark matter distribution, and overall evolutionary pathways that have shaped the Milky Way over billions of years.