Galaxies & the Milky Way

Chapter 6 surveyed our own Solar System. This chapter zooms out to the much larger real structure that Solar System sits inside — the Milky Way — and the broader real system astronomers use to classify galaxies in general.

The Hubble Sequence: Real Galaxy Classification

Edwin Hubble published a real morphological classification scheme for galaxies in 1926, dividing them into three broad classes: elliptical galaxies (smooth, featureless light distributions, appearing as ellipses), spiral galaxies (a flattened disk with spiral arms around a central bulge), and irregular galaxies (no consistent structure at all). A fourth real category, lenticular galaxies (S0), sits at the classification's own midpoint, intermediate between ellipticals and spirals. Spirals themselves split further into regular spirals (S) and barred spirals (SB) — roughly half of all real spiral galaxies have a bar-like structure extending from the central bulge.

The Milky Way: Real Structure and Scale

The Milky Way is itself a real barred spiral galaxy, with a standard measured diameter of about 87,400 light-years (26.8 kiloparsecs) — though its real dark matter halo may extend the galaxy's own effective structure out to nearly 2 million light-years, far beyond the visible disk. Current real estimates place its total star count between 100 and 400 billion.

Our Own Real Address

The Sun sits about 27,140 light-years from the Milky Way's own galactic center, positioned near the inner rim of the Orion Arm (also called the Orion Spur) — one of the galaxy's own major spiral-arm structures, itself a smaller feature branching between two of the Milky Way's larger arms.

Sagittarius A*: Real Evidence for a Supermassive Black Hole

At the Milky Way's own center sits Sagittarius A*, a real supermassive black hole with a measured mass of 4.297 ± 0.012 million solar masses, per 2023 data from the GRAVITY collaboration. Reinhard Genzel and Andrea Ghez shared a quarter each of the real 2020 Nobel Prize in Physics for discovering, through decades of tracking individual stars' own real orbits around the galactic center, that Sagittarius A* had to be a supermassive compact object — with a black hole as the only real remaining explanation. Roger Penrose received the other half of that same prize for earlier theoretical work showing black holes were a real, robust prediction of general relativity. On 12 May 2022, the Event Horizon Telescope Collaboration released the first real direct image of Sagittarius A* itself — based on radio interferometer data collected in 2017, requiring five years of real computational processing before the image could actually be produced.

The Andromeda Collision: A Real, Recently Revised Prediction

Real Milky Way Vital Statistics

Hands-On Exercises