Classify galaxies as elliptical, spiral or irregular, describe the Milky Way and our place in it, and explain how light and telescopes let us see galaxies

Learning goal: Define a galaxy and identify the three main types; describe the Milky Way as a spiral galaxy and locate the solar system within it; use the light-year to express distances and explain why seeing a galaxy means seeing its past; explain Hubble's law and redshift; explain that galaxies formed when gravity collapsed gas clouds into stars; and reason about which wavelengths reach the ground and where a telescope should be placed.

What a galaxy is. A galaxy is a group of stars, from a few thousand in the smallest dwarf galaxies to trillions in the largest, along with star systems, star clusters, gas and dust, all held together by gravity. Galaxies are the large building blocks of the universe: there are billions of them, and they are separated by distances far larger than their own size. They are classified by size and shape. Most observed galaxies fall into three types. Elliptical galaxies are smooth, round or oval collections of stars with little gas and dust and few new stars forming. Spiral galaxies have a flattened disk with arms winding out from a central bulge; the arms are rich in gas and dust and hold most of the young stars. Irregular galaxies have no regular shape and are often small and gas-rich, with active star formation. The type is read from images; the reasons one type forms rather than another involve collisions and history and are not the focus of this course.

The Milky Way. The faint band of light across a dark night sky is the disk of our own galaxy seen from inside. The Milky Way is a spiral galaxy with more than 100 billion stars and a diameter of more than 100,000 light-years. At its center is a massive black hole surrounded by a bulge of stars that swells above and below the disk; from the bulge extend spiral arms of gas, dust and most of the galaxy's young stars. The solar system lies in the disk, well away from the center, at a distance of very roughly 26,000 light-years; that figure is approximate. Our Sun is one ordinary star among the hundred billion, and every star you can see with the unaided eye belongs to the Milky Way.

The light-year. Distances to galaxies are so large that kilometers become unmanageable, so astronomers use the light-year: the distance light travels in one year. Light moves at about 300,000 km/s, and a year has about 31.6 million seconds, so one light-year is about 9.5 trillion kilometers, 9.5 × 10¹² km. A light-year is a distance, not a time, although it is defined using one. The nearest large spiral galaxy to ours, the Andromeda galaxy, is about 2.5 million light-years away, and the Milky Way is more than 100,000 light-years across, so the Milky Way is roughly 25 times narrower than the gap to Andromeda.

Seeing a galaxy means seeing its past. Because light takes time to travel, the light entering your eye from Andromeda tonight left it about 2.5 million years ago. You see the galaxy as it was then, not as it is now. For galaxies billions of light-years away, telescopes record light that set out billions of years ago, when the universe was younger. This is why deep images are a kind of time machine and why observing distant galaxies tests the model of the universe's history. It also means "how far?" and "how long ago?" are closely linked: a light-year is a unit of distance, and for galaxies out to a few hundred million light-years the light-travel time in years is about the same number as the distance in light-years. For very distant galaxies the two numbers differ, because space expands while the light is traveling.

Why we can see galaxies at all. Stars emit light; a galaxy of a hundred billion stars is bright enough that its light crosses millions of light-years and still reaches us, though spread so thin that most galaxies are invisible without a telescope. Telescopes gather far more light than an eye and can record it for hours. Different instruments collect different parts of the electromagnetic spectrum: radio telescopes map cold gas, infrared telescopes see through dust into star-forming regions, visible-light telescopes show stars, and x-ray telescopes reveal the hottest gas and the surroundings of black holes. Earth's atmosphere lets through visible light and radio waves but blocks x-rays, gamma rays, almost all ultraviolet and much infrared, so telescopes for those wavelengths must fly above the atmosphere.

A worked model: which telescope to fund. (This scenario is adapted from a classroom example in Ohio's science model curriculum.) Two proposals reach a funding panel. Project 1 is an ultraviolet telescope on a mountain at 14,000 feet, meant to study distant galaxies. Project 2 is a visible-light telescope in orbit, meant to observe one pair of nearby binary stars. Reasoning from what the atmosphere blocks: ozone in the stratosphere, mostly 10 to 40 km up and far above any mountain, absorbs almost all of the shortest-wavelength ultraviolet (UV-C) and about 95 percent of UV-B, so mainly the longest ultraviolet wavelengths, next to violet, reach the ground. So project 1 would collect little of the ultraviolet light it is designed for and should be moved to orbit or redesigned; visible light passes through the atmosphere, and a bright nearby binary pair can be studied from the ground at a small fraction of the cost of a satellite, so, unless the binary is too close for ground-based telescopes to separate, project 2 buys little for a great deal. As written, neither proposal has made its case, and a sound recommendation is conditional. It says what information is missing (which ultraviolet wavelengths project 1 needs, since the near-ultraviolet next to violet does reach a mountain top; how close together the binary stars are and how sharp the images for project 2 must be), what answer would change the verdict, and what change would fix each proposal. Placement and wavelength must match, and cost matters because space telescopes are far more expensive to build and cannot easily be repaired.

A telescope above the atmosphere. The Hubble Space Telescope, launched into orbit in 1990, was built to observe from above the blurring and absorption of the atmosphere in visible, ultraviolet and near-infrared light. Its main mirror was found to have been ground to a slightly wrong shape, and astronauts installed corrective optics in a 1993 servicing mission; later missions replaced instruments and worn parts. Its sharp images of distant galaxies, and its measurements of their distances, have been central to refining the expansion rate of the universe. Hubble's upgrades were all made by expensive crewed Space Shuttle missions; the engineering lesson is that repairing an instrument in space is difficult and costly, which is why designs are tested exhaustively on the ground.

Hubble's law and redshift. Light from nearly every galaxy beyond our local group is shifted toward the red end of the spectrum. Redshift is explained in this course as a Doppler shift: when a source and an observer move apart, the waves arriving are stretched to longer wavelengths, which for visible light means toward red. Hubble's law states that galaxies farther away have larger redshifts, so a galaxy's recession speed is proportional to its distance from us. This is the same relationship used as evidence of expansion in the previous lesson, and the Doppler effect itself is met again in the waves part of this course.

How galaxies formed. In the early universe, gravity acted on slight unevenness in the gas of hydrogen and helium. Denser regions pulled in more gas, collapsed, and fragmented into stars; the stars, bound together by their mutual gravity, made up the first galaxies. Galaxies have since grown by pulling in gas and by merging with one another. The stars in a galaxy orbit its center, held by the combined gravity of everything in it.

Investigation: a scale model of the Milky Way and its neighbor. Materials: a long tape measure, chalk, a playing field or hallway, a calculator. Steps: choose a scale of 1 m for 10,000 light-years; draw the Milky Way as a disk 10 m across with a central bulge 1 m across; mark the Sun's position 2.6 m from the center; measure and mark where Andromeda would be, 250 m away, with a second 10 m disk if space allows. Record the scale, each distance in light-years and in meters, and the ratio of the gap to the disk size. A result that supports the claim: the two disks are separated by about 25 times their own width, so galaxies are far apart compared with their size, yet nearer to each other than stars are, relative to size. What the model cannot show: the thickness of the disk, the spiral arms' true form, the black hole, or the motion of the galaxies; and the Sun on this scale would be far too small to draw.

Worked example 1 — classifying from a description. Three galaxies are described. Galaxy P: a flat disk with a bright central bulge and two arms winding outward, bluish young stars and dark dust lanes along the arms. Galaxy Q: a smooth, featureless oval of yellowish light with no visible dust and no bright young stars. Galaxy R: a ragged patch of stars and glowing gas with no disk, bulge or symmetry. P is a spiral: the disk, bulge, arms, dust and young stars are the signature. Q is elliptical: smooth shape, little gas and dust, older stars. R is irregular: no regular shape and active star formation in gas. The Milky Way, seen from outside, would look like P.

Worked example 2 — converting a distance into light travel time. One light-year is the distance light travels in a year, so for a nearby galaxy the number of light-years is also, to a good approximation, the number of years the light has been traveling (the light-year itself stays a distance unit). Andromeda is about 2.5 million light-years away, so its light took about 2.5 million years to arrive: we see it as it was 2.5 million years ago. A galaxy about 65 million light-years away is seen by light that left it about 65 million years ago, close to the time the large dinosaurs died out on Earth (about 66 million years ago). For much more distant galaxies the distance in light-years and the travel time in years are no longer the same number, because space expands while the light is on its way, so astronomers quote the light-travel time itself. Compare times with times: light that has traveled for 2 billion years left its galaxy when the Sun, which formed 4.6 billion years ago, already existed; light that has traveled for 6 billion years set out before the Sun existed. Reading light-years as "years ago" is a shortcut for nearby galaxies only, out to a few hundred million light-years.

Worked example 3 — the size of a light-year. Light travels about 300,000 km each second. Seconds in a year: 365.25 days × 24 hours × 3600 seconds ≈ 31.6 million seconds. Distance: 300,000 km/s × 31,600,000 s ≈ 9.5 × 10¹² km, about 9.5 trillion kilometers. The Milky Way's diameter of more than 100,000 light-years is therefore more than 9.5 × 10¹⁷ km. Nobody works in kilometers at this scale, which is why the light-year exists.

Worked example 4 — placing a telescope. An astronomer wants to observe the hot gas around a black hole, which emits mainly x-rays. Earth's atmosphere absorbs x-rays completely, so no mountain is high enough; the telescope must orbit above the atmosphere. A second astronomer wants to map cold hydrogen gas in spiral arms using radio waves. Radio waves pass through the atmosphere, so a large dish on the ground works and costs far less. The rule: match the telescope's location to the wavelength the atmosphere lets through, and spend on space only when the ground cannot do the job.

Representation task: a galaxy comparison and a scale sketch. Make a table with rows for elliptical, spiral and irregular galaxies and columns for overall shape, presence of a disk and arms, amount of gas and dust, whether young stars are forming, and one example (the Milky Way and Andromeda are spirals; leave the other rows as descriptions if you cannot name an example with confidence). Then draw the Milky Way from above at a stated scale, label the bulge, the disk, the spiral arms, the central black hole and the Sun's approximate position about 26,000 light-years from the center, and write beside it the galaxy's diameter (more than 100,000 light-years) and star count (more than 100 billion).

Independent performance: a telescope funding recommendation. Write a one-page recommendation to a funding panel choosing between the two proposals described in the lesson: an ultraviolet telescope on a 14,000-foot mountain to study distant galaxies, and a visible-light telescope in orbit to observe one nearby binary star pair. Success criteria: for each proposal, the recommendation states which wavelengths the atmosphere transmits and blocks and whether the chosen site suits the chosen wavelength; it weighs the far greater cost of space telescopes and the difficulty of repairing them; it explains what science is gained or lost in each case; it gives a clear recommendation, which may be conditional or may be to fund neither as written, names any information the proposals leave out (such as the exact ultraviolet wavelengths needed, or how close together the binary stars are), and gives a specific change that would make each proposal sound; and it uses the terms light-year, redshift or spectrum correctly at least once each where relevant.