Galaxies are vast cosmic islands of stars, gas, dust, and dark matter held together by gravity. Hubble's keen eye has revealed intricate details of the shapes, structures, and histories of galaxies - whether alone, as part of small groups, or within immense clusters. From supermassive black holes at galactic centers to giant bursts of star formation to titanic collisions between galaxies, these discoveries allow astronomers to probe the current properties of galaxies as well as examine how they formed and developed over time.
Astronomers classify galaxies into three major categories: elliptical, spiral and irregular. These galaxies span a wide range of sizes, from dwarf galaxies containing as few as 100 million stars to giant galaxies with more than a trillion stars.
Ellipticals, which account for about one-third of all galaxies, vary from nearly circular to very elongated. They possess comparatively little gas and dust, contain older stars and are not actively forming stars anymore. The largest and rarest of these, called giant ellipticals, are about 300,000 light-years across. Astronomers theorize that these are formed by the mergers of smaller galaxies. Much more common are dwarf ellipticals, which are only a few thousand light-years wide.
Question 1: How does the passage describe galaxies, and what holds them together?
Answer: The passage describes galaxies as vast cosmic islands of stars, gas, dust, and dark matter held together by gravity.
Question 2: What specific details about galaxies has Hubble revealed, according to the passage?
Answer: Hubble has revealed intricate details of the shapes, structures, and histories of galaxies, including information about supermassive black holes, bursts of star formation, and collisions between galaxies.
Question 3: How do astronomers classify galaxies, and what are the characteristics of each type?
Answer: Astronomers classify galaxies into three major categories: elliptical, spiral, and irregular. Elliptical galaxies vary in shape from nearly circular to very elongated, contain older stars, and have comparatively little gas and dust. Spiral galaxies have well-defined spiral arms and ongoing star formation. Irregular galaxies lack a defined shape and may result from interactions between galaxies.
Question 4: What is the range of sizes observed for galaxies, and what are some examples mentioned in the passage?
Answer: Galaxies span a wide range of sizes, from dwarf galaxies containing as few as 100 million stars to giant galaxies with more than a trillion stars. Examples mentioned in the passage include giant elliptical galaxies, which can be up to 300,000 light-years across, and dwarf elliptical galaxies, which are only a few thousand light-years wide.
Question 5: What distinguishes giant elliptical galaxies from dwarf elliptical galaxies?
Answer: Giant elliptical galaxies are much larger in size, up to 300,000 light-years across, compared to dwarf elliptical galaxies, which are only a few thousand light-years wide. Additionally, giant ellipticals are theorized to form from the mergers of smaller galaxies, while dwarf ellipticals are more common and contain fewer stars.
Question 6: How do the properties of galaxies, such as their shapes and star-forming activity, provide insights into their formation and development?
Answer: The shapes, structures, and star-forming activity of galaxies allow astronomers to probe their current properties and examine how they formed and developed over time. For example, elliptical galaxies with older stars and little ongoing star formation suggest they formed through mergers and have evolved differently from spiral galaxies with active star formation.
