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Category Archives: 15. The Milky Way: our home in the Universe

Astronomers from the International Centre for Radio Astronomy Research (ICRAR), in Perth in Western Australia, have produced what they call the GLEAMoscope to enable you to view the Milky Way over many different wavelengths, from gamma-ray to radio. (The name ‘GLEAM’ is derived from GaLactic and Extragalactic All-Sky Murchison Widefield Array.)


Credit: Natasha Hurley-Walker/GLEAM team


The plane of the Milky Way is shown as the horizontal across the middle. The north pole of the Galaxy is towards the top. Go to the GLEAMoscope website and use the sliding bar at the top left of the to change the wavelengths shown. (Note the image above is just a still and is not interactive.)  Different wavelengths reveal different features, from the dull red glow of hydrogen gas permeating through space, to the stars and dust clouds of our visible galaxy, and superhot gas generating x-rays.

Link: GLEAMoscope.

From an ESA press release, September 14, 2016 :

The first catalogue of more than a billion stars from ESA’s Gaia satellite was published on September 14, 2016 – the largest all-sky survey of celestial objects to date.

On its way to assembling the most detailed 3D map ever made of our Milky Way galaxy, Gaia has pinned down the precise position on the sky and the brightness of 1.142 billion stars. As a taster of the richer catalogue to come in the near future, this data release also features the distances and the motions across the sky for more than two million stars.


Credit: ESA/Gaia/DPAC

The map projection above shows an all-sky view of stars in the Milky Way and our neighboring galaxies, based on the first year or so of Gaia’s observations. It shows the density of stars observed by Gaia in each portion of the sky. Brighter regions indicate denser concentrations of stars, while darker regions correspond to patches of the sky where fewer stars are observed. Darker regions across the Galactic Plane correspond to dense clouds of interstellar gas and dust that absorb starlight along the line of sight. Many globular and open clusters – groupings of stars held together by their mutual gravity – are also sprinkled across the image.

Note that the faint curved features and dark stripes are not of astronomical origin but rather reflect Gaia’s scanning procedure. As this map is based on observations performed during the mission’s first year, the survey is not yet uniform across the sky. These artefacts will gradually disappear as more data are gathered during the five-year mission.

Links: ESA press release, Gaia sky map.

From a press release of the European Southern Observatory (ESO), September 23, 2015:

A new image of the rose-colored star forming region Messier 17 was captured by the Wide Field Imager on the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory in Chile. It is one of the sharpest images showing the entire nebula and not only reveals its full size but also retains fine detail throughout the cosmic landscape of gas clouds, dust and newborn stars.

Credit: ESO

Credit: ESO

Although officially known as Messier 17, its nicknames include: the Omega Nebula, the Swan Nebula, the Checkmark Nebula, the Horseshoe Nebula and the Lobster Nebula. M17 is located about 5500 light-years from Earth near the plane of the Milky Way and in the constellation of Sagittarius. The object spans a big section of the sky — its gas and dust clouds measure about 15 light-years across. This material is fueling the birth of new stars and the wide field of view of the new picture reveals many stars in front of, in, or behind M17.

The nebula appears as a complex red structure with some graduation to pink. Its coloring is a signature of glowing hydrogen gas. The short-lived blue stars that recently formed in Messier 17 emit enough ultraviolet light to heat up surrounding gas to the extent that it begins to glow brightly. In the central region the colors are lighter, and some parts appear white. This white color is real — it arises as a result of mixing the light from the hottest gas with the starlight reflected by dust. Throughout this rosy glow, the nebula shows a web of darker regions of dust that obscure the light. This obscuring material is also glowing and — although these areas are dark in this visible-light image — they look bright when observed using infrared cameras.

Links: full ESO press release, including further images and movies of M17.

From a press release from the Chandra X-ray Center and NASA’s Marshall Space Flight Center, September 23, 2015:

Three orbiting X-ray space telescopes have detected an increased rate of X-ray flares from the usually quiet giant black hole at the center of our Milky Way galaxy after new long-term monitoring. Scientists are trying to learn whether this is normal behavior that was unnoticed due to limited monitoring, or these flares are triggered by the recent close passage of a mysterious, dusty object.

Credit: NASA/CXC/MPE/G.Ponti et al; Illustration: NASA/CXC/M.Weiss

Credit: NASA/CXC/MPE/G.Ponti et al; Illustration: NASA/CXC/M.Weiss

By combining information from long monitoring campaigns by NASA’s Chandra X-ray Observatory and ESA’s XMM-Newton, with observations by the Swift satellite, astronomers were able to carefully trace the activity of the Milky Way’s supermassive black hole over the last 15 years. The supermassive black hole, Sagittarius A*, weighs in at slightly more than 4 million times the mass of the Sun. X-rays are produced by hot gas flowing toward the black hole.

The new study reveals that Sagittarius A* (Sgr A* for short) has been producing one bright X-ray flare about every ten days. However, within the past year, there has been a ten-fold increase in the rate of bright flares from Sgr A*, at about one every day. This increase happened soon after the close approach to Sgr A* by a mysterious object called G2.

Originally, astronomers thought G2 was an extended cloud of gas and dust. However, after passing close to Sgr A* in late 2013, its appearance did not change much, apart from being slightly stretched by the gravity of the black hole. This led to new theories that G2 was not simply a gas cloud, but instead a star swathed in an extended dusty cocoon.

While the timing of G2’s passage with the surge in X-rays from Sgr A* is intriguing astronomers see other black holes that seem to behave like Sgr A*. Therefore, it’s possible this increased chatter from Sgr A* may be a common trait among black holes and unrelated to G2. For example, the increased X-ray activity could be due to a change in the strength of winds from nearby massive stars that are feeding material to the black hole.

If the G2 explanation is correct, the spike in bright X-ray flares would be the first sign of excess material falling onto the black hole because of the cloud’s close passage. Some gas would likely have been stripped off the cloud, and captured by the gravity of Sgr A*. It then could have started interacting with hot material flowing towards the black hole, funneling more gas toward the black hole that could later be consumed by Sgr A*.

Links: Full Chandra press release; detailed image description; MNRAS paper by G. Ponti et al.

Here is a consolidated list of errors from the text’s first printing. Many of these have already been posted here as separate chapter updates. (Our publisher will make the necessary corrections to the printed book at the earliest opportunity.):

p. 25, Figure It Out 2.3: The last paragraph (about Fraunhofer) shouldn’t be there. Instead, it should be at the end of the caption of Figure 2-4 on p. 26.

p. 64, Q34: 1 Angstrom should be listed as 1010 m, not 108 m.

p. 64, Q41: Ditto

p. 78: There is an error in the equation relating the apparent magnitude and brightness of stars in Figure It Out 4.1.  In this equation, 2.512 should be raised to a power equal to (mB−mA).

p. 92, Q1: We could more clearly say “On the top picture” instead of just “On the picture” – since there are now two pictures on the opening page of the chapter (and the stars are somewhat too dense for individual clarity in the bottom picture).

p. 190: First sentence of Section 7.4d: “a little larger” should be “a little smaller” for the relative sizes of Triton and the Moon.

p. 309, Q53, there is a printing error when going from the bottom of column 1 to the top of column 2. At the top of column 2, the “(e)” should be boldface, there should be a period after “1/16”, and the remainder of the text should be deleted.

p. 363, column 1, second line from the bottom: When referring to the event horizon: “1/3” should be “2/3”, i.e. the sentence should read “Its radius is exactly 2/3 times that of the photon sphere…”

p. 391, column 2, second line from the bottom: for Spitzer, “Section 3.8c, Figure 3-32a).” should say simply “Section 3.8c).”

p. 392, column 1, line 5: At the end, add “(See Section 3.8c, Figure 3-32a.)”

Appendix 3C, column 3 header: “105 km” should be “106 km”

From a press release of the Max Planck Institute for Radio Astronomy, April 21, 2015:

Joint observations by the Atacama Pathfinder Experiment (APEX) telescope in Chile and Antarctica’s largest astronomical telescope come closer to making detailed images of the supermassive black hole in the centre of the Milky Way, up to its very edge, the “event horizon”. These successful observations were conducted within the framework of the Event Horizon Telescope – a virtual telescope as big as planet Earth.

Event Horizon Telescope

Credit: © Dan Marrone/University of Arizona

The South Pole Telescope (SPT) and APEX joined together in a ‘Very Long Baseline Interferometry’ experiment for the first time in January 2015. The two telescopes together observed two sources — the black hole at the centre of the Milky Way galaxy, Sagittarius A*, and the black hole at the centre of the nearby galaxy Centaurus A — and combined their signals to synthesize a telescope 7,000 kilometres across. With this success, the SPT joins the Event Horizon Telescope array, which connects APEX, the Large Millimeter Telescope in Mexico, the Submillimeter Telescope in Arizona, the Combined Array for Research in Millimeter-wave Astronomy in California, the Submillimeter Array and James Clerk Maxwell Telescope in Hawaii, and the Institute for Radio Astronomy Millimetrique (IRAM) telescopes in Spain and France.

Even though the Milky Way’s black hole is 4 million times more massive than the Sun, it is tiny to the eyes of astronomers. Smaller than Mercury’s orbit around the Sun, yet almost 26,000 light years away, studying its event horizon in detail is equivalent to standing in New York and reading the date on a cent in Germany.

With its unprecedented resolution, more than 1,000 times better than the Hubble Telescope, the EHT will see swirling gas on its final plunge over the event horizon, never to regain contact with the rest of the universe. If the theory of general relativity is correct, the black hole itself will be invisible because not even light can escape its immense gravity. However, it might still be seen as a silhouette against the background.

Links: MPIfR press release; EHT home.

Adapted from a press release of the Max Planck Institute for Extraterrestrial Physics, November 24, 2014:

Astronomers at the Max Planck Institute for Extraterrestrial Physics recently presented new observations of the gas cloud G2 in the Galactic Centre, which was originally discovered in 2011. These data are in remarkably good agreement with an on-going tidal disruption. As a complete surprise came the discovery that the orbit of G2 matches that of another gas cloud detected a decade ago, suggesting that G2 might actually be part of a much more extensive gas streamer. This would also match some of the proposed scenarios that try to explain the presence of G2. One such model is that G2 is originating from the wind from a massive star.

The gas cloud G2 is on a highly eccentric orbit around the Galactic Center. Observations in 2013 have shown that part of the gas cloud is already past its closest approach to the black hole, at a distance of roughly 20 light hours (a bit more than 20 billion kilometres).

The new, deep infrared observations with the SINFONI instrument at the VLT track the ongoing tidal disruption of the gas cloud by the powerful gravitational field. While the shape and path of the gas cloud agrees well with predictions from the models, so far there has been no significant enhanced high-energy emission, as one might have expected from the associated shock front.

Copyright and credit: Max Planck Institute for Extraterrestrial Physics

Copyright and credit: Max Planck Institute for Extraterrestrial Physics

However, a closer look into the data set led to a surprise. A decade ago, another gas cloud – now call G1 – was observed in the central region of our galaxy and it has a similar orbit. The researchers postulate that G1 and G2 might be clumps of the same gas streamer. G1 and G2 could be clumps in the wind ejected from of one of the massive disk stars in the vicinity. This could help to explain the missing X-ray emission from the gas cloud near the black hole (although the non-detection of such emission is not yet understood).

Links: MPE press release, including figures and detailed captions.

Adapted from a UCLA press release, November 3, 2014.

For years, astronomers have been puzzled by a bizarre object in the center of the Milky Way that was believed to be a hydrogen gas cloud headed toward our galaxy’s enormous black hole. (See Section 15.5, Chapter opener figure, p. 382, and Figure 15-5, p. 388.)

Having studied it during its closest approach to the black hole this summer, UCLA astronomers believe that they have solved the riddle of the object widely known as G2.

A team led by Andrea Ghez determined that G2 is most likely a pair of binary stars that had been orbiting the black hole in tandem and merged together into an extremely large star, cloaked in gas and dust – its movements choreographed by the black hole’s powerful gravitational field. The research is published today in the journal Astrophysical Journal Letters.

Astronomers had figured that if G2 had been a hydrogen cloud, it could have been torn apart by the black hole, and that the resulting celestial fireworks would have dramatically changed the state of the black hole. However, G2 survived and continues on its orbit unaffected.

G2 appears to be just one of an emerging class of stars near the black hole that are created because the black hole’s powerful gravity drives binary stars to merge into one. In our galaxy, massive stars primarily come in pairs. The star suffered an abrasion to its outer layer but otherwise will be fine.

Keck Observatory

Credit and copyright: Ethan Tweedie Photography

The team utilized the Keck Observatory’s adaptive optics technology, a powerful technology that corrects the distorting effects of the Earth’s atmosphere in real time to more clearly reveal the space around the supermassive black hole.

Links: full UCLA press release, Keck press release.

Adapted from a press release of the Royal Astronomical Society (RAS):

A team of international astronomers has created a detailed three-dimensional map of the dusty structure of the Milky Way, as seen from Earth’s northern hemisphere.

3D map, detail

Credit: Sale et al./IPHAS

Dust and gas, which make up the interstellar medium (ISM), fill the space between stars in galaxies. The dust in the ISM is shaped by turbulent flows that form intricate fractal structures on scales ranging from thousands of light years down to hundreds of kilometers. Rather than measuring the dust itself to create the map, the team has used observations of more than 38 million stars to estimate how much starlight has been obscured by the ISM and thus how much dust lies in our line of sight to each star. This ‘extinction’ map derives from the newly released catalog of the Isaac Newton Telescope Photometric H-alpha Survey of the Northern Galactic Plane (IPHAS), the first digital survey to cover the entire northern Milky Way.

The map shows how extinction builds with distance away from the Sun (typically out to 12,000 light years or more) in any part of the surveyed northern Milky Way. The fractal nature of the ISM is visible in the map, as are large-scale features, such as star-forming molecular clouds and bubbles of ionized gas around clusters of hot stars.

Links: Full RAS press release; IPHAS homepage for map downloads (large file sizes) c/o Stuart Sale, University of Oxford.

In Chapter 15 (the opening photo, p. 382, and Figure 15-15d, p. 395), we discuss the prospective effect of a gas cloud called G2 (“G” for “gas”) that was heading for the center of the Milky Way, perhaps dropping material in to the supermassive black hole known as Sagittarius A* (pronounced A-star) and causing it to flare brightly in x-rays and radio waves, at least. But the prediction for its closest approach is about now, mid-2014, and no brightening has apparently happened. It is still possible that there could be dramatic flaring in the future, but that could be years or decades off.

Credit: ESO

Credit: ESO

Scientists at the Max-Planck Institute for Extraterrestrial Physics in Germany base it on their observations with the European Space Agency’s Very Large Telescope. They suggest that “G2 may be a bright knot in a much more extensive gas streamer.”

Daryl Haggard, who has recently moved to Amherst College from Northwestern University, is lead author of a report of Chandra X-ray Observatory monitoring of “Sgr A*/G2” through six observations in the first half of 2014, including the predicted time of the closest encounter.

These articles describing the situation is available free online, and the main results are discussed by correspondent Ron Cowen in The New York Times for July 22, 2014.

Links: NY Times article by Cowan; the original ApJ article by Oliver Pfuhl, Stefan Gillessen, and a dozen others; Daryl Haggard’s report, from The Astronomer’s Telegram.