Question

Which is a group of stars, gas, and dust held together by gravity? A. constellation C. black hole B. supergiant D. galaxy

Short answer

D. galaxy

Step-by-step solution

Understand the Question

The question asks us to identify which among the given options describes a group of stars, gas, and dust held together by gravity.

Option A - Constellation

A constellation is a pattern or grouping of stars that is recognized in the sky. However, constellations are not necessarily held together by gravity; they are simply visual patterns as seen from Earth.

Option B - Supergiant

A supergiant is a type of star that is very large and luminous. It does not describe a group of stars, gas, and dust held together by gravity.

Option C - Black Hole

A black hole is a region in space with a gravitational pull so strong that nothing, not even light, can escape it. It does not describe a group of stars, gas, and dust.

Option D - Galaxy

A galaxy is a system of millions or billions of stars, together with gas and dust, that is bound together by gravitational forces.

Conclusion Based on Options

From the options given, only a galaxy fits the description of a group of stars, gas, and dust held together by gravity.

Key concepts

Galaxies

Galaxies are vast collections of stars, gas, and dust, all bound together by the invisible yet powerful force of gravity. These enormous structures contain millions, billions, or even trillions of stars, each surrounded by their own solar systems and residing within various types of galaxies, such as spiral, elliptical, and irregular galaxies.
Spiral galaxies, like our own Milky Way, are characterized by their majestic, swirling arms that extend outward from their centers, housing young and hot stars. Elliptical galaxies, on the other hand, tend to be more rounded and contain older stars, with less new star formation occurring. Irregular galaxies lack a definitive shape, often appearing chaotic.

• Spiral Galaxies: recognizable by their spiral arms.
• Elliptical Galaxies: round and contain ancient stars.
• Irregular Galaxies: without a clear structure.

All galaxies are held together by gravity, creating stunning cosmic landscapes across the universe from nearby neighbors to those billions of light-years away.

Gravity

Gravity is the invisible force responsible for binding together the cosmic structures that make up our universe, including stars, planets, galaxies, and even larger assemblies. This fundamental force pulls objects toward one another and is the reason why galaxies remain intact, despite being composed of so many dynamic elements.
Gravity is not uniform everywhere; instead, it depends on the mass of the objects and the distance between them, as defined by Newton's Law of Universal Gravitation: \[ F = G \frac{m_1 m_2}{r^2} \] - where \( F \) is the force of gravity, \( G \) is the gravitational constant, \( m_1 \) and \( m_2 \) are the masses, and \( r \) is the distance between the centers of the two masses.
In a galaxy, gravity acts to hold stars in orbit around the galactic center, similar to how planets orbit the sun within our solar system. Without gravity, galaxies would simply scatter into the vastness of space, leaving a very different universe than the one we observe today.

Stellar Patterns

Stellar patterns, known commonly as constellations, are groups of stars forming recognizable shapes as seen from Earth. These stars often appear connected, but they are not physically bound or held together by gravitational forces. They are merely line-of-sight groupings that have helped humans navigate and tell stories for centuries.

• Ancient cultures across the globe developed stories based on these stellar patterns.
• Astronomers use constellations to document and categorize different parts of the night sky.

With changing seasons and Earth's rotation, different constellations become visible, yet their apparent shapes remain a constant cultural reference that links us to our past. Remember, though compelling as they are, they do not represent physically associated clusters like those found in a galaxy.

Cosmic Structures

Cosmic structures encompass a wide array of large-scale formations in the universe, including galaxies, star clusters, and superclusters. These structures are interconnected across vast cosmic distances, forming a complex web sometimes referred to as the "cosmic web." This web is a framework within which matter clumps under the influence of gravity, creating intricate patterns and voids across the universe.
Star clusters, smaller than galaxies, are collections of stars that are grouped closely together due to gravity. There are two main types of star clusters: open clusters, which contain a few thousand stars, and globular clusters, which can house hundreds of thousands of stars in a spherical shape.
Superclusters contain groups of galaxy clusters and are among the largest-known structures in the universe.

• Open Clusters: loose formations of a few thousand stars.
• Globular Clusters: densely packed, spherical groups of stars.
• Superclusters: enormous collections of galaxy clusters.

Cosmic structures provide the grand stage on which the stories of galaxies and stars unfold, painting a breathtaking picture of the universe's organization and evolution.