Question

Which of these lists of cosmic features is in the correct order of size, beginning with the largest? a) Universe, galaxy, star, planet b) Star, galaxy, universe, planet c) Universe, planet, star, galaxy d) Galaxy, universe, star, planet

Short answer

Option (a) is the correct order: Universe, galaxy, star, planet.

Step-by-step solution

Identify the Largest Cosmic Feature

First, recognize that the largest cosmic feature among those listed is the Universe. The Universe encompasses everything, including galaxies, stars, and planets.

Determine the Next Largest Feature after the Universe

Next, determine the feature that is one level smaller than the Universe, which is the galaxy. Galaxies contain billions of stars.

Identify the Third Largest Feature

Within a galaxy, stars are the next largest feature. A single star can have planets orbiting it.

Identify the Smallest Feature

Finally, the smallest feature in the list is a planet. Planets orbit around stars.

Verify the Correct Order

Check which list option matches the determined correct order from largest to smallest: Universe, galaxy, star, planet. Option (a) 'Universe, galaxy, star, planet' matches this order.

Key concepts

Universe

The Universe is the grandest entity that encompasses everything we know. It contains all existing matter, energy, planets, stars, and galaxies. Imagine the Universe as an immense, all-encompassing space that is everything we see and do not see around us.
Everything that exists, from the tiniest particles to the largest galaxies, is part of the Universe.
It has been expanding since the Big Bang, which is the event that scientists believe created it.
Some features of the Universe include:

• Immeasurable size: The Universe is believed to be around 13.8 billion years old, and it is constantly expanding.
• Contains billions of galaxies: Each galaxy has its own formation of stars and planets.
• A subject of scientific research: The Universe holds many mysteries still under exploration by astrophysicists.

Galaxy

A galaxy is a massive system made up of stars, star systems, star clusters, dust, and gas all bound together by gravity.
Galaxies come in different shapes and sizes, including spiral, elliptical, and irregular forms.
They are fundamental building blocks of the Universe. Within a galaxy:

• Stars form the key elements: Often numbering in the billions within each galaxy.
• Planets may orbit stars: These celestial bodies are smaller but essential to the galaxy's makeup.
• Galactic center: Many galaxies, including the Milky Way, have a supermassive black hole at their center.

Typically, galaxies are categorized by their appearance. For example, our own galaxy, the Milky Way, is a barred spiral galaxy, home to our solar system and billions of other stars.

Star

Stars are luminous spheres of plasma held together by gravity. They are one of the most fundamental components of galaxies. Stars are incredibly hot and emit light and heat due to nuclear fusion occurring at their cores.
The process primarily involves the fusion of hydrogen atoms into helium, releasing immense energy.
Key characteristics of stars:

• Range vastly in size, brightness, and temperature.
• Our Sun is a star: It is the closest star to Earth and the center of our solar system.
• Lifecycle stages: Stars evolve from stellar nurseries, moving through stages such as main sequence, red giant, and potentially ending as a black hole or neutron star.

Stars form constellations in the night sky, and their life cycles influence the formation and evolution of planets and other astronomical bodies.

Planet

Planets are celestial bodies that orbit stars and are bound by gravity. They do not produce light on their own but reflect the light of their star. In a galaxy, planets are abundant and come in various forms such as terrestrial (rocky like Earth), gas giants (like Jupiter), or ice planets (like Neptune).
Planets can have moons, rings, and various atmospheres.
Characteristics of planets include:

• Orbits a star or stellar remnant.
• Massive enough to be rounded by its gravity, but not so massive as to initiate nuclear fusion (which distinguishes them from stars).
• Cleared their neighboring region of planetesimals, which are smaller bodies that could be part of a planet's formation.

Planets are part of solar systems and can host life forms, with Earth being the only known planet so far to support life. Each planet has unique properties dictated by its composition, distance from its star, and presence of moons.