Hubble observes awesome star fields

Hubble ACS SWEEPS Field

Recently, NASA released two spectacular photos taken by the Hubble Space Telescope, showing very dense star fields in our own Milky Way galaxy and in the Andromeda Galaxy.

The photo above shows the Sagittarius-I Window “in the crowded central bulge of our galaxy, 26,000 light-years away”. [1]

The size of the field of view on the sky is roughly that of the thickness of a human fingernail held at arm’s length, and within this region, Hubble sees about a quarter million stars towards the bulge. [1]

I highly recommend you check out the larger versions of this photo, to fully appreciate the extraordinary quantity of stars.

This particular area was first observed by Hubble in 1994 (the image was released in 1998), and again in 2006 during its SWEEPS survey (Sagittarius Window Eclipsing Extrasolar Planet Search).

The Sagittarius-I Window (also known as SGR-I) is a “low-extinction window” – a region containing relatively little interstellar dust, allowing an unusually clear view into the heart of our galaxy. The astronomer Walter Baade first identified this area in his 1963 book “Evolution of Stars and Galaxies”, in which he also identified other similar areas, including the Sagittarius-II Window (SGR-II) and the area now known as Baade’s Window (BW).

The annotated photo below shows the locations of Baade’s Window and the SWEEPS survey area (the Sagittarius-I Window):

Sagittarius Area with Location of SWEEPS

Here are the ICRS coordinates of all three windows (as given by the SIMBAD astronomical database), along with links for locating them in WikiSky. The SWEEPS survey area has slightly different coordinates to SGR-I, so I’ve included it as a separate entry.

Area ICRS coordinates WikiSky
RA Dec V1 V2
SWEEPS field 17h 59m -29° 12′ View View
Sagittarius-I Window 17h 56m -29.0° View View
Sagittarius-II Window 18h 12m -27.9° View View
Baade’s Window 18h 3m -28.0° View View

The number of stars in the SWEEPS survey area is stunning, but now take a look at this photo!

Star Field in M31 Imaged by Hubble WFC3

This is the area surrounding the Cepheid variable star V1 in the Andromeda Galaxy, about 2.5 million light-years away. The number of stars which can be seen in the larger versions of this photo is truly mind-blowing!

The stars look like grains of sand, and many of them have never been seen before. [2]

As Dave Bowman would say:

“Oh my God – it’s full of stars!” Yellow smiley

The illustration below indicates the location of this star field within the galaxy:

Hubble Views the Star that Changed the Universe

The circled star is the Cepheid variable star V1, discovered by the astronomer Edwin Hubble in 1923. The brightness of a Cepheid variable star oscillates with a very regular period, and because the star’s luminosity is directly related to its oscillating period, the distance to the star can be determined fairly accurately. When Hubble calculated the distance to V1, he realised it was too far away for it to be part of our own Milky Way galaxy. Astronomers had been debating the size of the universe around this time, and Hubble’s discovery proved that the Andromeda Galaxy is a large independent galaxy, not a small nebula within our own galaxy.

Images are not copyrighted. Credit for images:

  • Milky Way: NASA, ESA, W. Clarkson (Indiana University and UCLA), and K. Sahu (STScI).
  • Annotated photo: NASA, ESA, Z. Levay (STScI) and A. Fujii.
  • Andromeda Galaxy: NASA, ESA, and the Hubble Heritage Team (STScI/AURA).
  • Illustration: NASA, ESA, Z. Levay (STScI), and the Hubble Heritage Team (STScI/AURA).

Image sources:


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