7 The Interstellar Medium and Birth of Stars

Astronomy 103

The Interstellar Medium and Birth of Stars

I. The Interstellar Medium

  • Definition: The Interstellar Medium (ISM) refers to the matter that exists in the space between stars, primarily gas and dust.

  • Density:

    • Interstellar space has around 1 atom/cm³.

    • Earth's atmosphere has 10²⁰ atoms/cm³.

    • Best vacuums on Earth contain about 1000 atoms/cm³.

  • Dust Grains: Very sparse, with about 1 grain/km³.

II. Phases of Gas in the ISM

  • Three Phases:

    • Cold Gas: Temperature around 10-100s K.

    • Warm Gas: Approximately 8000 K.

    • Hot Gas: Reaches temperatures of 10^6 K.

  • Most volume in ISM is hot gas, but the majority of gas is warm or cold.

  • Significance of Cold Gas: Cold gas gathers into clouds that are crucial for star formation.

III. Types of Interstellar Clouds

  • HII Regions:

    • Formed by stars heating the gas, ionizing hydrogen.

  • HI Regions:

    • Cooler clouds where hydrogen is in atomic or molecular form.

IV. Visibility of Clouds

  • Bright Hot Clouds: Emit visible light.

  • Cool Clouds: Emit little light, seen as dark nebulae, blocking light from background stars.

    • Dust scatters shorter wavelengths, causing reddening of light.

V. Dust Properties

  • Dust grain size: typically 100s nm, comparable to visible light wavelengths.

  • Scattering Effects: Different colors affected differently by dust; hot dust scatters and absorbs light differently.

VI. Reddening Effects

  • Barnard 68: A specific example showcasing light reddening due to dust.

    • Similar effects are observed in sunsets.

VII. Types of Nebulae

  • Reflection Nebula: Appears blue due to scattering of blue light (similar to the blue sky).

    • Light scattered from stars depends on the surrounding dust.

  • Emission Nebula:

    • Produced by hot stars ionizing their surroundings, resulting in bright emissions.

VIII. Star Formation Process

A. Overview of Star Formation

  • Stars form from gas and dust clouds collapsing under gravity.

  • Understanding is still developing, but the process is simplified into stages.

B. Stages of Star Formation

  1. Stage I: Molecular Cloud Collapse

    • A fragment of a molecular cloud begins to collapse.

  2. Stage II: Cloud Fragmentation

    • Collapse leads to multiple cores forming in the cloud, which contract and gain mass from infalling gas.

  3. Stage III: Cooling Cores

    • Cores remain cold while collapsing, allowing radiation to cool them down. As density increases, heating begins.

  4. Stage IV: Formation of Protostar

    • A hot core forms, considered a protostar surrounded by a gas disk.

  5. Stage V: Protostellar Evolution

    • Protostar contracts, heats up but dims due to reduction in size. Jets of gas may also be expelled during this phase.

  6. Stage VI/VII: Star Birth

    • Once hydrogen fusion starts, collapse halts, marking the zero-age main sequence (ZAMS). Stars maintain their zones on the ZAMS but eventually move from it.

    • Some stars do not attain enough mass to start hydrogen burning and become brown dwarfs.

IX. Star Clusters

  • Stars typically form in groups called star clusters.

    • Open Clusters: Ranging from hundreds to thousands of stars (e.g., Pleiades, Hyades).

    • Stellar Associations: Less massive than open clusters, not gravitationally bound.

    • Globular Clusters: Larger clusters, containing millions of stars and are older.

X. Evolution of Clusters

  • Massive stars in clusters can disrupt stellar nurseries due to their ionizing radiation and stellar winds.

  • Most clusters eventually dissolve due to:

    • Loss of gas reducing gravitational binding.

    • Galactic gravity pulling stars apart.

  • Only massive structures like globular clusters tend to remain intact.

A star like the Sun stays on the main sequence for about **10 billion years**. During this phase, it undergoes hydrogen fusion in its core, which provides the energy needed to maintain its stability. After spending this duration in the main sequence, it will eventually evolve into a red giant before shedding its outer layers and ultimately becoming a white dwarf.