Galaxy Types, the Milky Way's Structure and Active Nuclei
Galaxies include spiral, elliptical and irregular types. The Milky Way is a barred spiral mapped from inside its disc. Active galactic nuclei release energy through gas accreting onto supermassive black holes; some nuclei outshine their host galaxy's stars.
What a learner can do afterwards
- Classifies a galaxy from described structural features and relates its type to typical stellar ages and cold-gas supply
- Explains how the Milky Way's spiral structure and central bar were mapped from a position inside the disc
- Uses a nucleus's luminosity and intrinsic variation time to constrain its emitting region and test an ordinary stellar explanation
1 · Read
Spirals have a rotating stellar disc with arms, often marked by gas and young blue stars. Their bulges usually contain older stars. Ellipticals have smooth, rounded outlines without arms. Their light is usually dominated by older stars, with little cold gas for new star formation. Irregulars lack either orderly shape and often have gas and young stars. These types are not a fixed sequence of life stages.
The Milky Way is a barred spiral whose stellar disc is about 100,000 light-years across. Its halo extends farther. The Sun lies in the disc, which contains older stars as well as gas and young stars. Recent star formation is concentrated near the disc plane and in the arms. Older stars dominate the bulge, the elongated central bar and the sparse stellar halo.
We map our galaxy from inside its disc. Dust hides distant stars in visible light. Radio emission traces gas through dust; infrared surveys reveal more stars because dust absorbs infrared less strongly. Combine a tracer's sky direction with a distance estimate to place it in the galaxy. Infrared star counts also reveal the elongated distribution of the bar.
In a density-wave model, an arm is a moving region of higher density. Stars and gas can pass through it instead of remaining there permanently. Gas compression can lead to new stars, whose blue light makes the arms stand out. Pattern motion and stellar orbits are different; the pattern need not move more slowly everywhere. Other models allow arms to change and reform. The same stars need not stay in an arm forever.
An active galactic nucleus is powered by gas accreting onto a supermassive black hole. Gas releases gravitational energy, heats up and radiates outside the event horizon. Some nuclei outshine their host's stars. If much of that light changes coherently within days, the varying region must be compact. That combination of high luminosity and compactness argues against an ordinary stellar population as the main engine.
For coherent intrinsic changes, estimate D as no greater than about c times t. D is the varying region's size; c is light speed; t is the change time in the source's frame. Ignore relativistic beaming here. A light-day is one day's travel at light speed. A six-day change gives an upper size limit of about six light-days, not an exact size. Rapid variation alone does not identify a black hole: stars can vary too.
Galaxy shape gives clues to stars and gas. Directions and distances map the Milky Way. A quasar's luminosity and rapid intrinsic variation support a compact accretion engine.
Lesson, questions and flashcards written for this dataset. Background facts were studied in OpenStax Astronomy, sections 25.1, 25.2, 26.2 and 27.2, and the American Museum of Natural History's What Is a Galaxy?
Readings and flashcards written for this dataset, with background facts studied in OpenStax Astronomy and the American Museum of Natural History's What Is a Galaxy? The linked video, What is the Milky Way Galaxy?, is credited to the American Museum of Natural History. Publisher terms for the video have not been verified.
More to read
Reading structure before colour
Start a galaxy classification with its structure. A disc with spiral arms indicates a spiral. An elliptical has a smooth, rounded outline without spiral arms. An irregular lacks either orderly shape.
Colour and gas provide additional clues. Young blue stars often make spiral arms conspicuous, but a spiral also contains older stars. Most of an elliptical's light comes from older stars, and typical ellipticals have little cold gas available for new star formation. Irregular galaxies can contain gas and stars of different ages.
A galaxy's type does not mark a fixed stage on a single timeline. Galaxies can change through mergers, other interactions or changes in their gas supply. Two galaxies with similar outlines need not have reached that appearance through the same history.
Where the Milky Way's stars live
The Sun belongs to the Milky Way's stellar disc. This disc is about 100,000 light-years across, while the halo reaches farther. The disc's diameter is therefore not the full extent of the galaxy.
The disc contains stars of many ages. Gas and recent star formation are concentrated near its plane, and young hot stars help trace the arms. Finding a star in the disc does not, by itself, establish that the star is young.
Older stars dominate the central bulge, the elongated stellar bar and the sparse stellar halo. Many globular clusters also belong to the halo. These components overlap rather than forming separate regions with sharp boundaries.
Separating an arm from its stars
An arm can remain visible without permanently holding the same stars. The useful distinction is between a pattern in the galaxy and the material that moves through it.
A density-wave model describes an arm as a moving region of higher density. Stars and gas can pass through that region. Gas compression can lead to star formation, and bright young blue stars make the arm easier to trace.
The pattern's motion is not identical to the orbital motion of each star. It should not be described as slower everywhere. Nor does one density-wave account settle every galaxy's spiral structure. Other models allow arms to change and reform.
An observed arm map records where tracers are concentrated. It does not demonstrate that each arm is a rigid object or a permanent group of stars.
Two measurements of an active nucleus
Luminosity and variation time answer different questions. Luminosity measures emitted power. A short variation time can constrain the size of the region whose light changes. Some active nuclei emit more light than the stars of their host galaxies.
Apply the light-travel estimate to a coherent, intrinsic change, using time in the source's own frame and ignoring relativistic beaming. A six-day change then implies a characteristic size no greater than about six light-days. This is an approximate upper bound on the varying region, not its exact size or the extent of the host galaxy.
A rapidly varying star is not a quasar. The challenge for an ordinary stellar population is to account for the compactness and the enormous luminosity together.
Gas accreting onto a supermassive black hole supplies an explanation. As the gas moves inward, gravitational energy is released and the gas heats and radiates. The observed light is emitted outside the event horizon.
Turning sky observations into a galactic map
A direction on the sky is not a complete position. Objects along one line of sight can be at different distances. Mapping the Milky Way therefore requires distance estimates as well as observations in many directions.
Dust obscures distant stars in visible light. Radio emission lets astronomers trace gas through the dust. Infrared surveys reveal more of the stars because dust absorbs infrared less strongly than visible light; dust still affects infrared.
Directions and distances place arm tracers in the galaxy. Infrared star counts help reveal the elongated distribution of the central bar. Gas and stars contribute different information, so astronomers combine several kinds of observations.
The result is a reconstruction made from within the disc. It is not a photograph taken from outside the Milky Way.
Explore more
2 · Watch
به انگلیسیTake it off screen
Where it sits
This opens up
Nothing builds on it yet.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.