Triangulum Galaxy
What is Triangulum Galaxy?
Galaxies· Distance ~2.7 million ly. The third-largest galaxy of the Local Group after Andromeda and the Milky Way. Its spiral arms face us almost head-on, and under truly dark skies it counts as the most distant object visible to the naked eye. It may be a satellite of Andromeda, and the two move in each other's grip.
📍 Visible from where you are right now — computed in your browser
bearing · altitude · rise time · best time tonight
Where Triangulum Galaxy is in your sky right now: https://nowsky.org/solar/sky?sel=triangulum galaxy — location and time set automatically, no login.
Its place in the sky
A galaxy, in the direction of Triangulum, spanning about 70 arcminutes of sky — 2.3 times the width of the full Moon.
Distance and scale
| Distance | ~2.7 million ly |
|---|---|
| Diameter | ~60,000 ly |
| Stars | ~40 billion |
| Constellation | Triangulum |
Figures are representative approximations. Source: compiled from NASA and IAU public data.
Discovery and name
Third-largest in the Local Group; Hubble's Cepheids here helped settle the 1920 Great Debate.
Frequently asked
- How far is Triangulum Galaxy from Earth?
- ~2.7 million ly.
- How big is Triangulum Galaxy?
- ~60,000 ly.
- Where in the sky is Triangulum Galaxy?
- In the direction of Triangulum.
- Where is Triangulum Galaxy in the sky right now?
- Open the sky view for your location and time and it shows the position in the sky right now.
A distance measured with water masers
In 2005 the distance to M33 was measured by a method that used no Cepheids. Water-molecule masers — natural microwave lasers — in the galaxy's star-forming regions were tracked for years with an intercontinental array of radio telescopes to measure the angular rate at which they move across the sky with the galaxy's rotation. Dividing by the true rotation speed from spectroscopy gives the distance: about 2.4 million light-years, the first purely geometric distance to a galaxy.
The method is valuable because it is an independent check that leans on no other rung of the cosmic distance ladder. The maser distance to M33 agreed with the Cepheid value (about 2.7 million light-years) to within roughly 10 percent, and the discrepancy itself became data on the ladder's errors. The same technique was applied to the galaxy NGC 4258 in the 2010s and became an anchor for measurements of the Hubble constant.
The galaxy in depth
The Triangulum Galaxy, M33, is a spiral about 2.7 million light-years away, a little beyond Andromeda. At around 60,000 light-years across it is just over half the Milky Way's size, with perhaps 40 billion stars. Its disc faces us almost squarely, so photographs show loosely wound arms in full — but that same orientation spreads the light thin, and the galaxy is trickier to see than its size suggests.
Messier listed it in 1764, and in the 1920s Hubble measured Cepheid variables here to fix its distance, helping settle that galaxies are separate 'island universes' beyond our own. It lies only about 750,000 light-years from Andromeda, close enough to be gravitationally bound, and a bridge of hydrogen between them hints at a past close encounter. Within it sits NGC 604, one of the largest emission nebulae in the Local Group.
On autumn evenings look below Andromeda, between Triangulum and Aries. Select the Triangulum Galaxy in the Sky view to mark its position; under a light-pollution-free sky it begins to show as a faint smudge in binoculars.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.