Small Magellanic Cloud

One of the farthest objects you can see with your bare eyes.


This view is a mozaic of 3 images, and spans 7° diagonally. The bright knot of stars is 47 Tucanae, the 2nd brightest globular cluster, after Omega Centauri. Most majestically of all is the diffuse glow of millions of stars, poorly resolved, that make up the bulk of the Small Magellanic Cloud. Some of the brightest nebulae and star clusters are highlighted in the image-over. 46 different NGC and IC catalog members are visible in this mozaic, most of them HII regions and open clusters.

Friendly extra-galactic neighbor, passing by.
Together with its larger counterpart the LMC these galaxies dominate the lower half of the sky in southerly direction. Viewed from Tivoli Southern Guest Farm, they are (barely) circumpolar, and roughly equidistant from the South Celestial Pole (SCP).
Unfortunately, their low declination means that for most of the inhabitants of Earth they do not rise very high above the horizon, if at all! Even from Tivoli they do not climb higher above the horizon than 40°. In order to observe them high in the sky, you'd have to travel to the southern tip of Chile or Argentine, where they rise to 70°.

For many decades it was believed that both the LMC and SMC were natural satellites of the Milky Way Galaxy. However, recent observations with the HST suggest both are moving too fast to be in orbit.

Size, Structure, and Distance
A study in 2004 1)Forty eclipsing binaries in the Small Magellanic Cloud: fundamental parameters and Cloud distance delivered the most precise distance to the SMC to date, by measuring the stellar parameters of 50 eclipsing OB-binaries. The study yielded precise estimates of the masses and spectroscopic class of the stars, which together with the detailed light curve provides a distance. The result is a mean distance of (60.6kpc +- 1.0kpc).
The SMC does not have a very well defined outer boundary, but the densest parts measures at least 2.6° x 2.0°, which translates to 2.8kpc x 2.1kpc linear size. This puts the SMC at roughly 1/10 the size of the Milky Way galaxy.

Optically there is about \(\) worth of mass, and roughly the same amount of invisible mass in the form of neutral gas.
Together with the LMC, it is the only galaxy to have its proper motion measured 2)Kinematics of the Magellanic System, at 1.45mas/yr +- 0.08mas/yr, which corresponds to a physical tangential velocity of about 416km/s. Together with the quoted radial velocity of 145km/s, the total relative movement (with respect to the sun) becomes 440km/s.

H-II Regions and clusters
Two large H-II regions can be readily identified, a bright cluster to the north and a lesser one to the east. The brightest ones are NGC 346 and NGC 371, featured in the image below.

NGC 346 contanins a bright central cluster, and the brightest recorded star of the SMC (HD5980). See high resolution photos (link,link).
probably measures up to about 400 ly across.

This image features another chain of H-II regions, the most prominent one being NGC456. It also shows some notable O-III emissions. Notice the semi-circular cluster of bright O-B stars. Closer inspection will reveal many smaller faint clusters without clear nebulosity, scattered all throughout the image region, such as these two ESO-labelled clusters.

Photography details
Relatively warm (10°C), very windy, observatory dome closed except for south fence to shield against strong wind gusts.
Field configuration:
Field 1: SE Corner, 21 subs (15x302s, 6x30s)
Field 2: 47 Tuc area, 24 subs (12x302s, 12x30s)
Field 3: SMC central, 20 subs (12x302s, 8x30s)

Field images are imperfect, focus drift evident. Field image processing done conventionally, flat field calibration, white balance, exponential histogram stretch. Mozaic custom tooling, chopping up image fields in smaller fragments, distortion correction, field solved, and per-channel re-combined on a TAN grid centered on SMC with 2.4px/arcsecond (slightly oversampled).

References   [ + ]

Image information

Galaxies, Globulars, Open clusters, Tivoli 2016