Want Astronomy Apps? There’s a Catalog for That

With the plethora of mobile apps now available for astronomy applications, it’s hard to keep track of them all. Thanks to astronomer Andy Fraknoi and the American Astronomical Society there’s now a catalog for that. “This catalog is a first attempt to make a list of those of particular interest to astronomy educators,” wrote Fraknoi.

The catalog, published by the Astronomy Education Review, includes a short description and reviews of some — but not all — the apps to help people distinguish which app will best cover their needs. However, “the number of apps is fast outpacing the ability of reviewers to keep up,” Fraknoi said, adding that suggestions and additions for this catalog are most welcome.

Click here to access the app catalog.

Properties of Black Holes

Since the Hubble Space Telescope was launched in 1990, there have been many observations of what are believed to be black holes, including the photograph below of a suspected black hole in the heart of the galaxy NGC 6251. But the subject of black holes began in theoretical physics, long before there were any observations by astronomers.

A black hole unobscured by dust

    The advent of Einstein's General Theory of Relativitygave physicists a mathematical language for describing the gravitational force in a manner consistent with the constant speed of light. Most of what we believe we know about black holes has come from abstract theoretical models in general relativity.
    But in order to observe black holes in Nature we need to know how those abstract theoretical models translate to a Universe filled with other stuff.

Abstract theoretical black holes

    In the abstract theoretical model of black holes, a black hole is studied as if it were the only thing in the Universe. Using that approximation, the math of general relativity becomes doable, and we can make predictions about black hole behavior that are useful in understanding the black holes we see. In addition, we learn a lot of things about black holes mathematically that we may never get a chance to witness directly through observation.
    In general relativity, the paths of light can be calculated for many different distributions of matter and energy using equations call thegeodesic equations. The geodesic equations give us the paths that would be followed by freely-falling test particles. For example, a baseball after being hit by Sammy Sosa and before being caught by an eager fan would be a freely falling particle, travelling on a geodesic path through spacetime.
    Light travels on geodesics paths through spacetime. When those geodesic paths cross the event horizon of a black hole, they never come back out. Interestingly, in a Universe where the energy density is never negative, this behavior of light leads mathematically to two very crucial properties of black holes:

• The surface area of the event horizon of a black hole can only increase, never decrease. This also means that although two black holes can join to make a bigger black hole, one black hole can never split in two.
• The pull of gravity at the event horizon is constant; it has the same value everywhere on the event horizon.

    Note that according to the first property, it is impossible for black holes to decay and go away, because a black hole cannot get smaller or split into smaller black holes. This is going to be changed when we add quantum mechanics to the theory in the next section.

Observable astrophysical black holes

    If a black hole traps all the light that crosses the event horizon, then how can we ever hope to observe one?
    In the abstract theoretical model of a black hole, it sits alone forever in the Universe letting us do math on it. In the Nature we observe, the Universe is filled with dust and gas in addition to stars, planets and galaxies. When dust and gas fall into a black hole, they can be sucked towards the event horizon so fast that the atoms are ionized and release bright light that escapes without crossing the event horizon.
    So the way astronomers and astrophysicists detect black holes in astronomical observations is to look for light from ionized dust and gas being sucked into something so fast that it could only be a black hole, not a normal gravitating massive object like a star.
    However, this bright light can be hard to see, because most black holes also attract giant clouds of interstellar dust that hide many of their features, as shown on the previous page. The suspected black hole shown in the photo above has a warped dust cloud around it, so that the bright light from the ionized gas can be seen.

2011MD

What did NOT happen

A newly discovered house-sized asteroid missed the Earth by less than 17,700 km (11,000 miles) yesteray, Monday June 27, 2011. That’s about 23 times closer than the Moon. The size and location of the asteroid, named 2011 MD, allowed observers in certain locations to take a look at the space rock, even with small telescopes. It’s closest approach was at 13:26 UTC on June 27.
A few hours before the asteroid's nearest approach, it appeared close to the sun, so observations were possible for only a brief period. Backyard astronomers were able to observe it with telescopes from Australia, southern Africa, and the Americas.
The asteroid was discovered on June 22, 2011, by the Lincoln Near-Earth Asteroid Research (LINEAR) pair of robotic telescopes in New Mexico, and according to rough estimates, the asteroid's length is between 10 and 45 meters (30 and 150 ft).
Emily Baldwin of Astronomy Now said that there was no threat of collision, and should the asteroid enter Earth's atmosphere, it would "mostly burn up in a brilliant fireball, possibly scattering a few meteorites", causing no likely harm to life or property on the ground.

Lunar eclipse

June 15th, 2011.

The lunar eclipse took place tonight, a central eclipse, with the Moon passing through the center of the Earth's shadow, which made the Moon appear very dark during the umbral (total) phase. Moreover, with the umbral phase lasting 100 minutes, this eclipse is among the longest eclipses that we will be seeing this century! By comparison, the longest lunar eclipse of this century happening on 27th July 2018 will be central and 103 minutes long.

The penumbral phase of the eclipse started at 7:25 PM and partial eclipse started at 8:23 PM. The Moon  lost its bright white color and slowly turned into a reddish/orangish color. Total eclipse began at 12:22 AM. The whole Moon was very dark and appeared a coppery red color for the 1 hour 40 minutes that the total phase lasted. Mid eclipse was reached at 10:13 PM and the eclipse will end at 1:01 AM.

Fortunately, where I live there were clear skies for the whole night. Unlike for solar eclipses, you were able to view it directly with the naked eye. If you are a photographer, send me your pictures and I will upload them here!

There is another lunar eclipse taking place at the end of the year on 10 December 2011. That eclipse starts at 4:34 PM and reaches mid eclipse at 4:32 PM, which doesn't make for as good viewing as the one this month.

Basic space-time explanation

It's hard to understand correctly the concept of space-time. Think of a very large ball. Even though you look at the ball in three space dimensions, the outer surface of the ball has the geometry of a sphere in two dimensions, because there are only two independent directions of motion along the surface. If you were very small and lived on the surface of the ball you might think you weren't on a ball at all, but on a big flat two-dimensional plane. But if you were to carefully measure distances on the sphere, you would discover that you were not living on a flat surface but on the curved surface of a large sphere.
    The idea of the curvature of the surface of the ball can apply to the whole Universe at once. That was the great breakthrough in Einstein's theory of general relativity. Space and time are unified into a single geometric entity called spacetime, and the spacetime has a geometry, spacetime can be curved just like the surface of a large ball is curved.
    When you look at or feel the surface of a large ball as a whole thing, you are experiencing the whole space of a sphere at once. The way mathematicians prefer to define the surface of that sphere is to describe the entire sphere, not just a part of it. One of the tricky aspects of describing a spacetime geometry is that we need to describe the whole of space and the whole of time. That means everywhere and forever at once. Spacetime geometry is the geometry of all space and all time together as one mathematical entity.
I'm preparing an entry about worm holes. I hope it'll be finished by tomorrow.

Black Hole Basic Understanding

Realest black hole picture I could find on the internet

Since the Hubble Space Telescope was launched in 1990, there have been many observations of what are believed to be black holes, including the photograph below of a suspected black hole in the heart of the galaxy NGC 6251. But the subject of black holes began in theoretical physics, long before there were any observations by astronomers.

    The advent of Einstein's General Theory of Relativity gave physicists a mathematical language for describing the gravitational force in a manner consistent with the constant speed of light. Most of what we believe we know about black holes has come from abstract theoretical models in general relativity.
    But in order to observe black holes in Nature we need to know how those abstract theoretical models translate to a Universe filled with other stuff.

In the abstract theoretical model of black holes, a black hole is studied as if it were the only thing in the Universe. Using that approximation, the math of general relativity becomes doable, and we can make predictions about black hole behavior that are useful in understanding the black holes we see. In addition, we learn a lot of things about black holes mathematically that we may never get a chance to witness directly through observation.
    In general relativity, the paths of light can be calculated for many different distributions of matter and energy using equations call the geodesic equations. The geodesic equations give us the paths that would be followed by freely-falling test particles. For example, a baseball after being hit by Sammy Sosa and before being caught by an eager fan would be a freely falling particle, travelling on a geodesic path through spacetime.
    Light travels on geodesics paths through spacetime. When those geodesic paths cross the event horizon of a black hole, they never come back out. Interestingly, in a Universe where the energy density is never negative, this behavior of light leads mathematically to two very crucial properties of black holes:

• The surface area of the event horizon of a black hole can only increase, never decrease. This also means that although two black holes can join to make a bigger black hole, one black hole can never split in two.
  
• The pull of gravity at the event horizon is constant; it has the same value everywhere on the event horizon.
  

Note that according to the first property, it is impossible for black holes to decay and go away, because a black hole cannot get smaller or split into smaller black holes. This is going to be changed when we add quantum mechanics to the theory. We'll look through that in my next post.

IC1805 - Heart Nebulae

A new mosaic from NASA’s newest infrared observatory captures the Heart and Soul nebulae, so named because of their resemblance to hearts — both the Hallmark-card and the blood-pumping variety.

“One is a Valentine’s Day heart, and the other is a surgical heart that you have in your body,” said Ned Wright of the University of California, Los Angeles, who presented the image May 24 at a meeting of the American Astronomical Society.

Since its launch Dec. 14, 2009, the Wide-Field Infrared Survey Explorer has been circling the Earth in a polar orbit and snapping images every 11 seconds. As of Sunday, it has captured 953,880 frames and mapped about 75 percent of the sky, Wright said.


The new image is stitched together from 1,147 individual frames. The exposure took a total of 3½ hours spread over 11 days in February to complete. The nebulae are located in the constellation Cassiopeia, about 6,000 light-years away from Earth.

The image is color-coded to make sense to human eyes, which are blind in the infrared. Blue and cyan represent the shortest wavelengths WISE is sensitive to — 3.4 and 4.6 micrometers — and highlight places where stars are being born. Green light shows small grains of dust that have been heated by starlight and glow at the 12-micrometer band. The longest wavelength, 22 micrometers, is shown in red, capturing larger dust grains.

The bright spot at the top right of the image is an active star-forming region called W3, which Wright studied with a 4-pixel balloon-borne telescope for his Ph.D. thesis in the 1970s. Wright marveled at the difference between the sketched-out contour map he made then and the glowing portrait captured by WISE.

“It’s been an amazing progress in IR astronomy, with cameras growing by a factor of a million in power in just a few decades,” he said.