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Monthly Archives: September 2015

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.

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From an article in The Economist, June 30, 2015:

During our summer in the northern hemisphere, we may get cloudless azure skies, some of the time at least. What makes the sky this colour?

For scientists, the answer is relatively straightforward: Rayleigh scattering. When white light from the Sun reaches the Earth it hits the gas molecules that make up the atmosphere. These molecules—mainly nitrogen and oxygen—are smaller than the wavelengths of light in the visible spectrum and so scatter the light. White light is made up of different wavelengths, which, since Isaac Newton’s experiments with prisms in the 17th century, we think of as a spectrum of seven different colors: red, orange, yellow, green, blue, indigo and violet. Light at the violet end of the spectrum travels in shorter, tighter waves, which are affected more by the molecules in the atmosphere than the longer, lower-frequency waves at the red end. This phenomenon is named after Lord Rayleigh, the British physicist who discovered it in the 19th century. The sky appears blue because shorter wavelengths are scattered more by the atmosphere than longer wavelengths; so the scattered sunlight that reaches our eyes when looking at the sky (rather than at the Sun itself) is predominantly blue.

Once you have got your head around that, it might seem that the answer to the initial question—“Why is the sky blue?”—is rather simple. But there is a catch: not everyone would agree that the sky is blue. In 1858 William Gladstone, better known for being the Prime Minister of Britain four times during the 19th century, published a treatise on Homer. He noted, with astonishment, that the Greek poet did not once use the word blue. He used color words rather oddly—he described the sea as “wine-dark”, iron as violet and honey as green. Further research showed that the Koran, the original Hebrew Bible, the Icelandic sagas and the Vedic hymns, written in India between 1500 BC and 1000 BC, also lack references to this hue, even when talking about the heavens. There are still many languages today that do not have a word that precisely correlates to the English word for the slice of the spectrum between green and purple. Russians might call the sky either goluboe (light blue) or sinee (darker blue); in Japan 青 (ao) encompasses the color of the sky but also apples and grass; the Namibian Himba tribe would describe the sky as zoozou, which roughly translates as “dark” and includes shades of red, green and purple as well as blue.

This is more than a pedantic issue of translation: evidence suggests that language has a huge impact on how people interpret the world. Incredible as it may seem, having a distinct word for a color reinforces and amplifies the perception of it as distinct from other shades. Without the word you don’t perceive it as readily. To prove this scientists showed groups of colored tiles to the Himba, who found it difficult to pick out one blue tile from a group of 11 green ones (although they found it far easier than English-speakers to spot one yellow-green tile hiding amongst some more pine-hued ones). So although it is true that to English-speakers the sky is blue, it is arguably only blue because they say it is.

Link: original article via http://www.economist.com.

From a HST press release, September 24, 2015:

A stunning new set of images from Hubble’s Wide Field Camera 3 capture the scattered stellar remains in spectacular new detail and reveal its expansion over the years since HST last captured them, in 1997.

Credit: NASA, ESA, Hubble Heritage Team

Credit: NASA, ESA, Hubble Heritage Team

Deriving its name from its delicate, draped filamentary structures, the beautiful Veil Nebula is one of the best-known supernova remnants. It formed from the violent death of a star twenty times the mass of the Sun that exploded about 8000 years ago. Located roughly 2100 light-years from Earth in the constellation of Cygnus (The Swan), this brightly coloured cloud of glowing debris spans approximately 110 light-years.

Astronomers suspect that before the Veil Nebula’s source star exploded it expelled a strong stellar wind. This wind blew a large cavity into the surrounding interstellar gas. As the shock wave from the supernova expands outwards, it encounters the walls of this cavity — and forms the nebula’s distinctive structures. Bright filaments are produced as the shock wave interacts with a relatively dense cavity wall, whilst fainter structures are generated by regions nearly devoid of material. The Veil Nebula’s colorful appearance is generated by variations in the temperatures and densities of the chemical elements present; they do not represent the real colors of the nebula.

Links: Full press release and description; images for download and video.

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.