August 18, 2017

Get ready! On Monday, August 21, if you live in the United States from Oregon to South Carolina, you will be able to experience one of nature’s most impressive sights–a total eclipse of the sun. Across the United States, large crowds are expected in towns, cities, and campsites along the path of totality for the spectacular celestial show. The path of totality is the narrow swath, about 70 miles (110 kilometers) wide, along which the moon will completely blot out the sun.

A solar eclipse occurs when the moon passes between Earth and the sun, blotting out the sun’s light. This photograph shows a total eclipse, in which the moon completely covers the face of the sun. The sun’s outer atmosphere, called the corona, appears as an irregularly shaped halo of light. Credit:

A total solar eclipse occurs when the Earth, sun, and moon are in nearly a straight line and the moon’s shadow sweeps across the face of Earth. The dark moon appears on the edge of the sun and moves slowly across. At the moment of totality, a brilliant halo flashes into view around the darkened sun. This halo is the sun’s outer atmosphere, the corona. The sky remains blue but darkens dramatically. Some bright stars and planets will become visible and the temperature will noticeably drop. After a few minutes, the sun reappears as the moon continues on its orbit. The period when the sun is totally darkened may be as long as 7 minutes 40 seconds, but it averages about 2 1/2 minutes.

A total eclipse of the sun, as shown here, starts at the left. The moon gradually covers the sun, shown photographed through a filter. At the time of the total eclipse, photographed without a filter, the sun’s corona (outer atmosphere) flashes into view. The sun reappears as the moon moves on. Credit: © Atlas Photo Bank/ Photo Researchers

In the United States, the path of totality will cross 14 states: Oregon, Idaho, Wyoming, Montana, Nebraska, Iowa, Kansas, Missouri, Illinois, Kentucky, Tennessee, Georgia, North Carolina, and South Carolina. This will be the first total solar eclipse to cross the United States from coast-to-coast since June 8, 1918, when a total solar eclipse darkened skies from Washington to Florida. The last total solar eclipse to be seen anywhere in the continental United States was in 1979.

Makanda, a village in southern Illinois just south of Carbondale, will see the longest duration of totality for the eclipse–about 2 minutes and 40 seconds. If you miss out, though, don’t worry. Another total solar eclipse will cross the same area in 2024!

If you are going to view the eclipse, be careful! Looking directly at the sun, even during an eclipse, can severely damage your eyes, even if you are wearing sunglasses. If you wish to look directly at the eclipse you will need “eclipse glasses” which have special solar filters. Make sure that your eclipse glasses are undamaged and meet safety standards. Be careful to look away from the sun when you put your eclipse glasses on and take them off. A total solar eclipse can be viewed safely without protection in the path of totality only during the brief time when the disk of the sun is completely hidden.

More on this story next week!

March 2, 2017

Time operates on an epic scale among the stars and galaxies of outer space. Some stars exist for millions of years but then suddenly undergo rapid changes and explode within months. In October 2013, an international team of scientists led by Ofer Yaron, an astrophysicist at the Weizmann Institute of Science in Israel, detected and studied a supernova that occurred in a distant galaxy within three hours of the explosion’s light first reaching Earth. Thanks to the timely observations, the team was able to learn a lot about the star and the explosion that consumed it. The team published its findings in February 2017 in the journal Nature Physics.

Supernova 1604 was a star that exploded in our own galaxy. The supernova blasted off the shell of gas and dust seen in this false-color composite image. The shell continues to expand at around 2,000 kilometers (1,200 miles) per second. The German astronomer Johannes Kepler observed the explosion in 1604. Credit: NASA/ESA/JHU/R. Sankrit & W. Blair

A supernova is an exploding star that can become billions of times as bright as our sun before gradually fading from view. At its brightest, a supernova may outshine an entire galaxy. The explosion throws a large cloud of gas into space at speeds of up to 10 percent of the speed of light, which is 186,282 miles (299,792 kilometers) per second. The mass of the expelled material may exceed 10 times the sun’s mass. Most supernovae reach their peak brightness in one to three weeks and shine intensely for several months.

A red supergiant called V838 Monocerotis glows at the center of a dust cloud in this photograph taken by the Hubble Space Telescope. In 2002, the star gave off a brilliant flash of light, becoming 600,000 times as bright as the sun. The flash illuminated dust thrown off the star during a previous outburst. Credit: NASA/ESA/H.E. Bond (STScI)

The exploding star examined by Yaron and his associates was a red supergiant. Such stars are dozens of times larger than our sun, which is a main sequence star or yellow dwarf. Red supergiants have relatively short life spans, however, existing for “only” millions of years. In contrast, our sun is expected to live some 10 billion years. All stars produce energy through the process of nuclear fusion, a joining of two atomic nuclei (cores) to produce a larger nucleus. Fusion releases a huge amount of energy. Most stars fuse hydrogen or helium, but a supergiant quickly (over millions of years) burns through its fuel supply and begins to fuse heavier elements together in its core. At this point, a supergiant’s days are numbered. Each new level of fusion chips away at its core, slowly killing the star. At a critical point, the star quickly fuses its available silicon into iron. Once the core fills with iron, the star will collapse and rebound in an explosive supernova.

Click to view larger image
A huge star creates chemical elements by nuclear fusion, the joining of two atomic nuclei to make a larger nucleus. In the outermost shell, hydrogen nuclei fuse, creating helium. In the next shell, helium fuses to make carbon and oxygen. Fusion creates successively heavier elements in shells closer to the core, where iron is produced. The shells in this diagram are not drawn to scale. Credit: WORLD BOOK diagram

Based on the patterns of light emitted by the supernova in question, Yaron and the team discovered that the star had blown off a layer of material into space about a year before the explosion. They suspect that this layer has to do with a change in fusion fuel at the star’s core shortly before it went supernova. The transition itself was violent, setting off a chain reaction within the star that shot a layer of star matter into space ahead of the supernova.

The discovery was made with the help of an ever-improving array of automated survey telescopes. Such telescopes capture images of a certain portion of the night sky. A computer then compares the images against earlier pictures of the same section of sky, looking for changes. If it detects any, the computer alerts a human astronomer to investigate the findings.

Astronomers are eagerly awaiting the next supernova to occur in the Milky Way. They estimate that a supernova occurs once every 100 years or so in our galaxy, but they are not always visible to Earth-bound observers. The last local supernova seen on Earth occurred in 1604, when German astronomer Johannes Kepler observed what he thought was a new star in the night sky. The most recent intragalactic (within our galaxy) supernova occurred around 1900, but its light was obscured by dust. Scientists were only able to study it 100 years later with instruments such as the orbiting Chandra X-Ray Observatory and the National Radio Astronomy Observatory’s Very Large Array near Socorro, New Mexico. If we’re lucky enough for the next supernova to be close (but not too close), we can learn more about the largest stars and the brilliant ends of their lives.

February 3, 2017

One of the greatest questions in the formation of the solar system is in our own planetary back yard: how was the moon made? The current hypothesis (proposed explanation)—that the moon formed from chunks of Earth unloosed in a massive collision—has held sway among planetary scientists for over 30 years. But as more is learned about the moon, scientists are exploring other possibilities, and three scientists in particular—Raluca Rufu and Oded Aharonson of the Weizmann Institute of Science, Israel, and Hagai B. Perets of the Technion Israel Institute of Technology—are offering a different explanation. They published their new theory last month in the journal Nature Geoscience.

A new theory suggests that the moon may have formed from debris unloosed by many small impacts on Earth rather than one big one. Credit: Lunar and Planetary Institute

Since the early days of astronomy, people have speculated on how the moon was formed. In the 1800’s, astronomers used to think that the moon split from Earth—but in a very peculiar way. The accepted hypothesis of that era said that in the distant past Earth spun so rapidly that a portion of it tore away, forming the moon and leaving behind a basin that became the Pacific Ocean. Scientists now know that plate tectonics formed the Pacific Ocean over hundreds of millions of years, and that Earth lacks the rotational speed to create such a spectacular split. In recent years, engineers have developed powerful computers that allow geologists to take new and closer looks at rocks returned from the Apollo moon landings from 1969 to 1972.

This artist rendering depicts the “Big Whack” hypothesis of Earth colliding with a planetary body. The resulting dust and debris from Earth would then have created the moon. (Credit: NASA/JPL-Caltech)

Since the 1980’s, one hypothesis has stood up best to scrutiny: that the moon formed as a result of a massive collision known as the Giant Impact or the “Big Whack.” According to this idea, a Mars-sized object collided with Earth about 4.6 billion years ago. As a result of the impact, a huge cloud of vaporized rock shot off Earth’s surface and went into orbit around Earth. The cloud cooled and condensed into a ring of small, solid bodies, which then gathered together, forming the moon.

If the Big Whack is favored, why are Rufu, Aharonson, and Perets exploring alternative ideas? The Big Whack explains many of the orbital and rotational characteristics of both Earth and the moon, but the hypothesis must be tweaked to an uncomfortable degree to account for the remarkable similarity of Earth rocks to moon rocks. The giant impactor would have had to have struck Earth in an extremely precise way to produce a moon with the makeup shown by returned lunar samples.

Therefore, the Israeli team started from scratch. They reasoned that because impacts were common in the early solar system, Earth should have been hit with objects large enough to create moons many times, not just once. They ran hundreds of computer simulations and found that a series of smaller impacts over the course of millions of years could explain the compositional similarity of Earth and its moon. A smaller body (more the size of the dwarf planet Ceres) would slam into Earth, forming a disk of debris that would eventually come together to form a moonlet, or mini-moon. Later, another body would collide with Earth, creating a new debris disk and another moonlet. Eventually, these moonlets would merge with one another. To reach the size of the current moon, a number of such collisions and moonlet creations and mergings (their guess was roughly 20) would be needed.

The new study is intriguing, but it does not disqualify the Giant Impact Hypothesis just yet. Rufu and her colleagues admit that much more research needs to be done to confirm their findings. For instance, the group did not determine if some of the moonlets could have been sucked back into Earth or flung out into the solar system. This would increase the number of impacts needed to make our moon, making this explanation less likely than a precise Giant Impact.

August 11, 2016

It is going to storm tonight—but don’t worry, you won’t get wet. The Perseid meteor shower peaks in the evening hours tonight and early tomorrow morning. Sky watchers in the Northern Hemisphere may be able to see bright streaks flashing across the night sky. Thanks to the mighty influence of the planet Jupiter, the Perseids are more numerous than they’ve been for several years.

The Perseid meteor shower lights up the night sky each year in August.
Credit: NASA/JPL

Meteor showers occur when Earth travels through a stream of meteoroids. A meteoroid is a piece of matter that enters Earth’s atmosphere at a high speed. The rate at which meteors appear generally increases, peaks, and then decreases as Earth moves in and out of a stream. As seen from the ground, all meteors in a particular shower appear to come from the same direction in the sky. This direction is called the radiant of the shower.

The Perseids are an annual meteor shower caused by a dusty trail of debris left by Comet Swift-Tuttle. As the comet orbits the sun, it sheds rock and dust particles. As Earth travels through this ring of debris, meteoroids crash into the atmosphere. Their high speed heats up the air, causing it to glow. Almost all these meteoroids are too small to reach Earth’s surface and instead burn up in the atmosphere. The Perseids get their name because the radiant of the shower lies near the constellation Perseus.

Comet Swift-Tuttle leaves a wide swath of debris, so meteors have been visible for several weeks already and should be visible for several more. The frequency of meteor sightings is expected to peak tonight, however, as Earth passes through the thickest part of the debris cloud. Astronomers estimate that there may be as many as 200 visible meteors per hour! What’s the reason for this abnormally heavy shower? The intense gravity of Jupiter sometimes tugs at debris streams left by comets. In the case of this year’s Perseids, Jupiter has pulled the center of the stream closer to Earth’s orbit, nearly doubling the usual meteor shower.

Look toward Perseus tonight after sunset to catch the show (it is best seen when the constellation is directly overhead). If possible, get away from urban light pollution, but some shooting stars can be seen even near a city’s glow. Astronomers think the brightest meteors may even outshine the stars tonight—for only a fraction of a second, of course!

Other World Book articles:

• Astronomy
• Famous comets
• Stones From Space (a Special report)

March 28, 2014

A probable dwarf planet that orbits much farther from the sun than any other known object has been discovered by American astronomers Chad Trujillo and Scott Sheppard. The little planet is so far away that its orbit takes it 15 times as far from the sun as the orbit of Neptune, the most distant major planet in the solar system. The astronomers first sighted the object, designated 2012 VP113, or VP for short, using the Dark Energy Camera mounted on the 4-meter Blanco telescope in Chile. However, to confirm the shape of the object’s orbit around the sun, the astronomers needed to watch the object for many months. The would-be dwarf planet VP is the second such object found orbiting the sun at such a vast distance. The first, named Sedna, was discovered in 2003 by a team that included Trujillo. VP appears to be about 280 miles (450 kilometers) across, about half the size of Sedna and the most commonly known dwarf planet, Pluto. Trujillo and Sheppard nicknamed the new dwarf planet Biden, for United States Vice President Joe Biden.

The distance from Earth to the sun is, on average, about 93 million miles (150 million kilometers). This distance is called an astronomical unit or AU. Neptune is about 30 AU from the sun. The orbit of Sedna is a stretched out oval that carries it as far as 1,000 AU from the sun. However, Sedna also comes to within 76 AU of the sun at its closest approach. The orbit of VP is more circular. For this reason, VP doesn’t stray as far as Sedna, only about 450 AU. On the other hand, VP never gets closer than 80 AU to the sun at its closest approach, farther than any other solar system body known.

Dwarf planets Ceres, Eris, Haumea, Makemake, and Pluto and their moons, compared with Earth and its moon. All of these dwarf planets are smaller than the moon. (©UCAR/University of Michigan)

The area to which Pluto and several other dwarf planets belong is called the Kuiper belt. It is the region beyond Neptune and includes millions of icy bodies. Far beyond the Kuiper belt is a region called the Oort cloud, an area thought to be filled with millions of comets. Both Sedna and VP orbit between the Kuiper belt and the Oort cloud. Many astronomers believed this area was empty. The discovery of Sedna and now VP puts that idea into question.

The discovery offers new information about how the solar system was formed. How did such large objects assume orbits so far from the sun? How many are there yet to be found? One interesting conclusion suggested by astronomers studying the new data is that the orbits of Sedna and VP may be influenced by a large, yet undiscovered planet.

Additional World Book articles:

• Ceres
• Eris
• Planet (the dwarf planets)
• Exploring the Suburban Solar System (a Special Report)
• Astronomy (2004) (a Back in Time article)
• Astronomy (2006) (a Back in Time article)

March 27, 2012

Sometimes things are not as simple as they appear. Take the planet Mercury, for instance. In a very short time, the smallest planet in the solar system (sorry Pluto, you’re no longer a planet) has gone from a simple hunk of rock, much like Earth’s moon, to one of the most unusual planets ever studied. For years, most astronomers thought that Mercury formed early in the solar system’s history and has since remained pretty much unchanged. New evidence shows a planet whose more recent history includes active volcanoes, a shifting crust, and other activities once thought to have ended billions of years ago. Some of these activities may result from the fact that Mercury is slowly shrinking. The shrinking would explain why the outer layer of the planet is buckling and cracking. A closer look at the surface also shows evidence of lava flows having occurred much more recently than previously thought.

The MESSENGER space probe has revealed surprising new findings about the smallest planet in the solar system. (Courtesy of NASA)

The planet’s surface is not the only surprise. Mercury may also have an internal structure like no other planet yet studied by scientists. Mercury appears to have a core consisting of three layers. Earth has only two such inner layers. Both Mercury and Earth have an inner core of solid metal surrounded by a second layer of liquid metal. Unlike Earth, Mercury appears to have a third layer made of solid iron and sulfur. Many scientists now speculate that this possible third layer of core explains one of the great mysteries concerning Mercury. Scientists have long wondered how the planet could be so small and yet have a much greater mass than the other inner planets–Venus, Earth, and Mars–for its size.

The new data on Mercury was collected by the MErcury Surface, Space ENvironment, GEochemistry and Ranging (MESSENGER) space probe. Launched in 2004 by NASA, MESSENGER began a one-year mission orbiting Mercury in early 2011 (which NASA scientists have now extended). The probe reached the planet after a more than five-year tour of the inner solar system. The long, looping route involved three close fly-bys of Mercury along with close approaches to Venus and Earth.

Just how loopy was the route MESSENGER took? Mercury is only about 20 percent farther from Earth than Mars when those planets pass closest to Earth. Yet, a probe going to Mars travels for less than a year. By comparison, MESSENGER took more than five years to reach Mercury.

Additional World Book articles

• Probing the Planets (a special report)
• Space Exploration 2004 (Back in Time article)
• Space Exploration 2008 (Back in Time article)
• Space Exploration 2009 (Back in Time article)
• Space Exploration 2011 (Back in Time article)