Another gorgeous image from Hubble! This close-up of NGC 7023, or the Iris Nebula, shows an area filled with cosmic dust. Illuminated from above by the nearby star HD 200775, the dust resembles pink cotton candy, accentuated with diamond-like stars. The “cotton candy” is actually made up of tiny particles of solid matter, with sizes from ten to a hundred times smaller than those of the dust grains we find on Earth, and the “diamonds” are both background and foreground stars.
The image was taken previous to Hubble’s recent servicing mission, using the Advanced Camera for Surveys. Astronomers also used Hubble’s Near Infrared Camera and Multi-Object Spectrometer (NICMOS) instrument to try to determine which chemical elements are present in the nebula.
NGC 7023 is a reflection nebula, which means it scatters light from the massive nearby star. Reflection nebulae are different from emission nebulae, which are clouds of gas that are hot enough to emit light themselves. Reflection nebulae tend to appear blue because of the way light scatters, but parts of the Iris Nebula appear unusually red-ish or pink.
Credit: X-ray: NASA/CXC/GSFC/M.Corcoran et al.; Optical: NASA/STScI
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Eta Carinae is a beast of a star. At more than 100 solar masses and 4 million times the luminosity of our Sun, eta Car balances dangerously on the edge of stellar stability and it’s ultimate fate: complete self-destruction as a supernova. Recently, Hubble Space Telescope observations of the central star in the eta Carinae Nebula have raised an alert on eta Car among the professional community. What they discovered was totally unexpected.
“It used to be, that if you looked at eta Car you saw a nebula and then a faint little core in the middle” said Dr. Kris Davidson, from the University of Minnesota. “Now when you look at it, it’s basically the star with a nebula. The appearance is completely different. The light from the star now accounts for more than half the total output of eta Car. I didn’t expect that to happen until the middle of this century. It’s decades ahead of schedule. We know so little about these very massive objects, that if eta Car becomes a supernova next Thursday we should not be very surprised.”
In 1843, eta Carinae underwent a spectacular eruption, making it the second brightest star in the sky behind Sirius. During this violent episode, eta Car ejected 2 to 3 solar masses of material from the star’s polar regions. This material, traveling at speeds close to 700 km/s, formed two large, bipolar lobes, now known as the Homunculus Nebula. After the great eruption, Eta Car faded, erupted again briefly fifty years later, then settled down, around 8th magnitude. Davidson picks up the story from there.
This light curve depicts the visual apparent brightness of Eta Car from 1822 to date. It contains visual estimates (big circles), photographic (squares), photoelectric (triangles) and CCD (small circles) observations. All of them have been fitted for consistency of the whole data. Red points are recent observations from La Plata (Feinstein 1967; Fernández-Lajús et al., 2009, 2010). Used by permission.
“Around 1940, Eta suddenly changed its state. The spectrum changed and the brightness started to increase. Unfortunately, all this happened at a time when almost no one was looking at it. So we don’t know exactly what happened. All we know is that by the 1950’s, the spectrum had high excitation Helium lines in it that it didn’t have before, and the whole object, the star plus the Homunculus, was gradually increasing in brightness. In the past we’ve seen three changes of state. I suspect we are seeing another one happening now.”
During this whole time eta Car has been shedding material via its ferocious stellar winds. This has resulted in an opaque cloud of dust in the immediate vicinity of the star. Normally, this much dust would block our view to the star. So how does Davidson explain this recent, sudden increase in the luminosity of eta Carinae?
“The direct brightening we see is probably the dust being cleared away, but it can’t be merely the expansion of the dust. If it’s clearing away that fast, either something is destroying the dust, or the stellar wind is not producing as much dust as it did before. Personally, I think the stellar wind is decreasing, and the star is returning to the state it was in more than three hundred years ago. In the 1670’s, it was a fourth magnitude, blue, hot star. I think it is returning to that state. Eta Carinae has just taken this long to readjust from its explosion in the 1840’s.”
After 150 years what do we really know about one of the great mysteries of stellar physics? “We don’t understand it, and don’t believe anyone who says they do,” said Davidson. “The problem is we don’t have a real honest-to-God model, and one of the reasons for that is we don’t have a real honest-to-God explanation of what happened in 1843.”
Can amateur astronomers with modest equipment help untangle the mysteries of eta Carinae? Davidson think so, “The main thing is to make sure everyone in the southern hemisphere knows about it, and anyone with a telescope, CCD or spectrograph should have it pointed at eta Carinae every clear night.”
Looking at a galaxy edge-on provides astronomers the opportunity to study different aspects of galaxies than a face-on view offers. This Hubble image of NGC 4710 is part of a survey conducted to provide more information about the puzzling bulges that form around the middle of some galaxies. Have these galaxies been “eating” too much, or is it just part of a “middle-age spread” similar to what humans experience? Astronomers aren’t sure why bulges evolve and become a substantial component of most spiral galaxies.
This image was taken by the Advanced Camera for Surveys in 2006, before the recent Hubble Servicing Mission.
A faint, ethereal “X”-shaped structure is also visible. Such a feature, which astronomers call a “boxy” or “peanut-shaped” bulge, is due to the vertical motions of the stars in the galaxy’s bar and is only evident when the galaxy is seen edge-on. This curiously shaped puff is often observed in spiral galaxies with small bulges and open arms, but is less common in spirals with arms tightly wrapped around a more prominent bulge, such as NGC 4710.
When targeting spiral galaxy bulges, astronomers often seek edge-on galaxies, as their bulges are more easily distinguishable from the disc. This exceptionally detailed edge-on view of NGC 4710 taken by the Advanced Camera for Surveys (ACS) aboard Hubble reveals the galaxy’s bulge in the brightly coloured centre. The luminous, elongated white plane that runs through the bulge is the galaxy disc. The disc and bulge are surrounded by eerie-looking dust lanes. A wide-field image of the region around NGC 4710 constructed from Digitized Sky Survey 2 data. The field of view is approximately 2.8 degrees x 2.9 degrees. Credit: NASA, ESA and Digitized Sky Survey 2. Acknowledgment: Davide De Martin (ESA/Hubble)
NGC 4710 is a member of the giant Virgo Cluster of galaxies and lies in the northern constellation of Coma Berenices (the Hair of Queen Berenice). It is not one of the brightest members of the cluster, but can easily be seen as a dim elongated smudge on a dark night with a medium-sized amateur telescope. In the 1780s, William Herschel discovered the galaxy and noted it simply as a “faint nebula”. It lies about 60 million light-years from the Earth and is an example of a lenticular or S0-type galaxy – a type that seems to have some characteristics of both spiral and elliptical galaxies.
Astronomers are scrutinizing these systems to determine how many globular clusters they host. Globular clusters are thought to represent an indication of the processes that can build bulges. Two quite different processes are believed to be at play regarding the formation of bulges in spiral galaxies: either they formed rather rapidly in the early Universe, before the spiral disc and arms formed; or they built up from material accumulating from the disc during a slow and long evolution. In this case of NGC 4710, researchers have spotted very few globular clusters associated with the bulge, indicating that its assembly mainly involved relatively slow processes.
The Wide Field and Planetary Camera 2, along with the “contact lens” that corrected the defect in the Hubble Space Telescope’s primary mirror will have a new home. Recently returned to Earth after more than 15 years in space, the two instruments will have a new home in the Smithsonian’s National Air and Space Museum in Washington. Astronauts on the Hubble servicing mission in May 2009 replaced WFPC-2 with a new and improved version, bringing the well-used camera back to Earth. “This was the camera that saved Hubble,” said Ed Weiler, from NASA’s Science Mission Directorate. “I have looked forward for a long time to stand in front of this very instrument while on display to the public.”
WFPC-2, and the Corrective Optics Space Telescope Axial Replacement, or COSTAR, gave Hubble the ability to take the images that have changed the way we see the Universe by providing the iconic images that now adorn posters, album covers, the Internet, classrooms and science text books worldwide.
The Hubble instruments will be on display in the National Air and Space Museum’s Space Hall through mid-December. They then will travel to Southern California to go on temporary display at several venues. In March 2010, the instruments will return to the Smithsonian Air and Space Museum, where they will take up permanent residency.
After Hubble’s launch and deployment aboard the shuttle in 1990, scientists realized the telescope’s primary mirror had a flaw, known as a spherical aberration. The outer edge of the mirror was ground
too flat by a depth of 2.2 microns, roughly equal to one-fiftieth the thickness of a human hair. This tiny flaw resulted in fuzzy images because some of the light from the objects being studied was scattered.
Hubble’s first servicing mission provided the telescope with hardware that basically acted as eye glasses. Launched in December 1993 aboard space shuttle Endeavour, the mission added the WFPC-2, about the size of a baby grand piano, and COSTAR, about the size of a telephone booth. The WFPC-2 had the optical fix built in, while the COSTAR provided the optical correction for other Hubble instruments.
The WFPC-2 made more than 135,000 observations of celestial objects from 1993 to 2009. The camera was the longest serving and most prolific instrument aboard Hubble.
“For years the Wide Field and Planetary Camera 2 has been taking pictures of the universe,” said John Trauger of NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “Today, we are taking pictures of the
WFPC-2 and I guess if there was ever a camera that deserves to have its picture taken, this is it.”
Carina Nebula. Image credit: Hubble Space Telescope/NASA.
When you look at the amazing pictures captured by the Hubble Space Telescope, or the Mars Exploration Rovers, do you ever wonder: is that what you’d really see with your own eyes? The answer, sadly, is probably not. In some cases, such as with the Mars rovers, scientists try and calibrate the rovers to see in “true color,” but mostly, colors are chosen to yield the most science. Here’s how scientists calibrate their amazing instruments, and the difference between true and false colors.
So, to start off, let’s put this in the form of a true or false question: T or F: When we see the gorgeous, iconic images from Hubble or the stunning panoramas from the Mars rovers, do those pictures represent what human eyes would see if they observed those vistas first hand?
Answer: For the Hubble, mostly false. For the rovers, mostly true, as the rovers provide a combination of so-called “true” and “false” color images. But, it turns out, the term “true color” is a bit controversial, and many involved in the field of extraterrestrial imaging are not very fond of it.
“We actually try to avoid the term ‘true color’ because nobody really knows precisely what the ‘truth’ is on Mars,” said Jim Bell, the lead scientist for the Pancam color imaging system on the Mars Exploration Rovers (MER). In fact, Bell pointed out, on Mars, as well as Earth, color changes all the time: whether it’s cloudy or clear, the sun is high or low, or if there are variations in how much dust is in the atmosphere. “Colors change from moment to moment. It’s a dynamic thing. We try not to draw the line that hard by saying ‘this is the truth!'”
Bell likes to use the term “approximate true color” because the MER panoramic camera images are estimates of what humans would see if they were on Mars. Other colleagues, Bell said, use “natural color.”
Zolt Levay of the Space Telescope Science Institute produces images from the Hubble Space Telescope. For the prepared Hubble images, Levay prefers the term “representative color.”
“The colors in Hubble images are neither ‘true’ colors nor ‘false’ colors, but usually are representative of the physical processes underlying the subjects of the images,” he said. “They are a way to represent in a single image as much information as possible that’s available in the data.”
True color would be an attempt to reproduce visually accurate color. False color, on the other hand, is an arbitrary selection of colors to represent some characteristic in the image, such as chemical composition, velocity, or distance. Additionally, by definition, any infrared or ultraviolet image would need to be represented with “false color” since those wavelengths are invisible to humans.
The cameras on Hubble and MER do not take color pictures, however. Color images from both spacecraft are assembled from separate black & white images taken through color filters. For one image, the spacecraft have to take three pictures, usually through a red, a green, and a blue filter and then each of those photos gets downlinked to Earth. They are then combined with software into a color image. This happens automatically inside off-the-shelf color cameras that we use here on Earth. But the MER Pancams have 8 different color filters while Hubble has almost 40, ranging from ultraviolet (“bluer” than our eyes can see,) through the visible spectrum, to infrared (“redder” than what is visible to humans.) This gives the imaging teams infinitely more flexibility and sometimes, artistic license. Depending on which filters are used, the color can be closer or farther from “reality.”
Stone mountain rock outcrop in true and false colour. Image credit: NASA/JPL
The same rock imaged in true and false color by Opportunity.
In the case of the Hubble, Levay explained, the images are further adjusted to boost contrast and tweak colors and brightness to emphasize certain features of the image or to make a more pleasing picture.
But when the MER Pancam team wants to produce an image that shows what a human standing on Mars would see, how do they get the right colors? The rovers both have a tool on board known as the MarsDial which has been used as an educational project about sundials. “But its real job is a calibration target,” said Bell. “It has grayscale rings on it with color chips in the corners. We measured them very accurately and took pictures of them before launch and so we know what the colors and different shades of grey are.”
One of the first pictures taken by the rovers was of the MarsDial. “We take a picture of the MarsDial and calibrate it and process it through our software,” said Bell. “If it comes out looking like we know it should, then we have great confidence in our ability to point the camera somewhere else, take a picture, do the same process and that those colors will be right, too.”
Hubble can also produce color-calibrated images. Its “UniverseDial” would be standard stars and lamps within the cameras whose brightness and color are known very accurately. However, Hubble’s mission is not to produce images that faithfully reproduce colors. “For one thing that is somewhat meaningless in the case of most of the images,” said Levay, “since we generally couldn’t see these objects anyway because they are so faint, and our eyes react differently to colors of very faint light.” But the most important goal of Hubble is produce images that convey as much scientific information as possible.
The rover Pancams do this as well. “It turns out there is a whole variety of iron-bearing minerals that have different color response at infrared wavelengths that the camera is sensitive to,” said Bell, “so we can make very garish, kind of Andy Warhol-like false color pictures.” Bell added that these images serve double duty in that they provide scientific information, plus the public really enjoys the images.
And so, in both Hubble and MER, color is used as a tool, to either enhance an object’s detail or to visualize what otherwise could not be seen by the human eye. Without false color, our eyes would never see (and we would never know) what ionized gases make up a nebula, for example, or what iron-bearing minerals lie on the surface of Mars.
As for “true color,” there’s a large academic and scholarly community that studies color in areas such as the paint industry that sometimes gets upset when the term “true color” is used by the astronomical imaging group, Bell explained.
“They have a well-established framework for what is true color, and how they quantify color,” he said. “But we’re not really working within that framework at that level. So we try to steer away from using the term ‘true color’.”
Levay noted that no color reproduction can be 100% accurate because of differences in technology between film and digital photography, printing techniques, or even different settings on a computer screen. Additionally, there are variations in how different people perceive color.
“What we’re doing on Mars is really just an estimate,” Bell said, “it’s our best guess using our knowledge of the cameras with the calibration target. But whether it is absolutely 100% true, I think it’s going to take people going there to find that out.”
For more information see http://hubblesite.org/ or check out Jim Bell’s 2006 book “Postcards From Mars.”
