Astronomy & Astrophysics 101: Spiral Galaxy

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The luminous heart of the galaxy M61 dominates this image, framed by its winding spiral arms threaded with dark tendrils of dust. As well as the usual bright bands of stars, the spiral arms of M61 are studded with ruby-red patches of light. Tell-tale signs of recent star formation, these glowing regions lead to M61’s classification as a starburst galaxy. Credit: ESA/Hubble & NASA, ESO, J. Lee and the PHANGS-HST Team

A spiral galaxy typically has a rotating disc with spiral ‘arms’ that curve out from a dense central region. The Milky Way is a spiral galaxy.A spiral galaxy typically has a rotating disc with spiral ‘arms’ that curve out from a dense central region. The Milky Way is a spiral galaxy.

Classifying spiral galaxies is not always straightforward, as their appearance varies considerably depending on their orientation relative to Earth. The most visually spectacular spiral galaxies are ‘face-on’, meaning that their bulge and all their spiral arms are clearly visible. The most challenging to identify are fully ‘side-on’, meaning that only the outer edge of one side of the galaxy’s arms is visible. The Milky Way is thought to be a barred spiral galaxy. Approximately 60% of all galaxies are thought to be spiral galaxies, making spiral galaxies the home of the majority of the stars in the Universe. Spiral galaxies are populated by stars that are on average much younger than those that populate elliptical galaxies, and current thinking suggests that spiral galaxies may evolve into elliptical galaxies. Spiral galaxies rotate, and their spiral shape is not stable. A puzzle of modern astronomy is how spiral galaxies maintain their spiral arms.

Spiral Galaxy. Credit: ESA/Hubble & NASA, J. Lee and the PHANGS-HST Team, Acknowledgement: Judy Schmidt

Hubble has captured beautiful images of the distinctive arms and spiral features of spiral galaxies throughout its more than 30-year history. Particularly popular is the Andromeda Galaxy — a large spiral galaxy — which Hubble has observed in unprecedented detail, capturing over 100 million stars and representing a new benchmark for precision studies of this galaxy type.