Question

Where in an H-R diagram are young stars located when they first become observable using visible light? Why aren't they visible at an earlier stage of their careers?

Short answer

Young stars first appear on the main sequence of the H-R diagram. Earlier, they are invisible due to surrounding gas and dust.

Step-by-step solution

Understanding the H-R Diagram

The Hertzsprung-Russell (H-R) diagram is a graphical tool that astronomers use to classify stars according to their luminosity, spectral type (temperature), and evolutionary stage. Generally, young observable stars are located on the main sequence of this diagram.

Identifying Young Observable Stars

When young stars first become observable in visible light, they are located on the main sequence of the H-R diagram. This is because they have started nuclear fusion in their cores, causing them to shine brightly and be detectable in the visible spectrum.

Stages Before Visibility

Before stars are visible in the H-R diagram, they are in earlier stages of their development, such as the protostar stage. During this time, they are embedded in dense clouds of gas and dust, and only emit light in the infrared spectrum, not visible light.

Understanding Why Early Stages Aren’t Visible

In the early protostar stage, the thick surroundings of gas and dust block visible wavelengths, preventing these stars from being seen. Once fusion begins, the added brightness and heat disperse the surrounding material, allowing the starlight to be observed with visible light.

Key concepts

Young Stars

Young stars are fascinating objects in the universe because they are just beginning their journey in the cosmic cycle of life. When we say a star is young, we're referring to the early stages of a star's life after it forms from a collapsing cloud of gas and dust.

These stars have recently started nuclear fusion in their cores, transforming hydrogen into helium, which marks the official "birth" of a star. At this stage, they are bright and can be seen in visible light.

• Young stars are typically found along the main sequence of the Hertzsprung-Russell (H-R) diagram.
• They have just enough mass and temperature to start nuclear fusion.
• Being on the main sequence is where they will spend the majority of their life span.

This phase is crucial as it determines how the star will evolve in the subsequent stages of its life.

Main Sequence

The main sequence is a continuous and distinctive band on the H-R diagram. It's where most stars, including our Sun, spend the majority of their lifespan.
When a star is on the main sequence, it is in a stable phase of its life where nuclear fusion occurs at a steady rate in the core, converting hydrogen into helium. This process releases a tremendous amount of energy, producing the star's light and heat.

• Stars on the main sequence range from hot, blue stars to cool, red stars.
• Their position on the main sequence depends mostly on their mass and temperature.
• This phase can last billions of years, depending on the star's mass.

The main sequence is a key indicator of a star's current state and provides insights into its size, temperature, and brightness.

Stellar Evolution

Stellar evolution refers to the life cycle of a star, from its formation to its demise. Understanding this process helps astronomers predict the future of stars and their effects on surrounding environments.
A star's life begins with the gravitational collapse of a gas and dust cloud, leading to various stages marked by dramatic transformations.

• Starting with the protostar stage, where the star is not yet visible to the naked eye.
• Progressing to the main sequence phase, where the star spends most of its life.
• Ultimately ending as a white dwarf, neutron star, or black hole, depending on its mass.

Stellar evolution is an ongoing process. It explains not only the passage of stars through different life stages but also how they interact with their surroundings, enriching galaxies with heavy elements.

Protostar Stage

The protostar stage is the earliest phase in the life of a star during which it is forming but not yet classified as a true star.
At this stage, the mass, still a dense clump of gas and dust, has begun to collapse under gravity. Although there is increasing pressure and heat, conditions are not yet sufficient for nuclear fusion.

• Protostars are often hidden in dense molecular clouds.
• They emit primarily in the infrared spectrum due to their heat, not visible light.
• This stage can last for hundreds of thousands of years.

Once a protostar reaches the necessary temperature and pressure for nuclear fusion, it disperses surrounding material and becomes visible in visible light. This transition marks the star's journey toward the main sequence on the H-R diagram.