Watch the sun’s great explosions

A solar filament that had been lurking atop the sun for a week finally exploded this month, the latest in a string of large solar explosions that NASA scientists say will peak in 2013. This explosion, seen below, released high-energy plasma into the solar system, but did not create auroras on Earth because it dispersed before reaching our atmosphere.

NASA's Solar Dynamics Observatory captured the image sequence, which shows the filament exploding. In August, NASA scientists observed a 28-hour period of shock waves, solar flare explosions and solar "tsunamis" that rocked the sun. They've named the event "The Great Eruption."

Scientists say the activity is a sign that the sun is "waking up" and heading for another "solar maximum" cycle in 2013, according to Space.com.

[See also: Where, how astronomers could find alien life]

"The August 1st event really opened our eyes," NASA scientist Karel Schrijver said. "We see that solar storms can be global events, playing out on scales we scarcely imagined before."

Scientists are still trying to figure out what kind of connection there is among the massive explosions that occurred so far apart on the sun's surface.

[Related: First 'alien' planet from another galaxy discovered]

"We're still sorting out cause and effect," Schrijver told the Daily Mail. "Was the event one big chain reaction, in which one eruption triggered another, bang, bang, bang, in sequence? Or did everything go off together as a consequence of some greater change in the sun's global magnetic field?"

[Photos: New view from space station window]

Watch video of the filament explosion:

Chart Watch Extra: Down To The Wire

Train's "Hey, Soul Sister" and "California Gurls" by Katy Perry featuring Snoop Dogg are locked in an extremely tight race to wind up as the song with the most paid downloads in 2010. With just three weeks to go in Nielsen/SoundScan's chart year, the two songs are just 2,000 units apart. "Hey, Soul Sister" has sold 4,154,000 digital copies so far this year. "California Gurls" has sold 4,152,000. Train's best hope of retaining its slim lead is that a quintessential summer song like "California Gurls" may have limited appeal in the closing weeks of December.

"Hey, Soul Sister" headed the year-to-date chart from the beginning of April through the end of October. "California Gurls" pushed ahead in the week ending Nov. 7. "Hey, Soul Sister" regains the lead this week. The song received two big boosts in recent weeks. It was featured in an episode of Glee on Nov. 30. The following night, Train performed it on the Grammy nominations special.

No matter which song winds up on top three weeks from now, "Hey, Soul Sister" is a bigger hit overall. It sold an additional 687K copies in 2009, which made it that year's #131 song.

"California Gurls" exemplified one of the year's hottest trends: the collaboration. Five of the top 10 songs so far this year (and 10 of the top 20) pair two artists who don't normally record together.

[Photos: Katy Perry's Crazy Costumes]

"Hey, Soul Sister" also exemplified one of the year's notable trends: the name-drop. The song mentions Madonna and Mr. Mister. "TiK ToK" gave a name-check to Mick Jagger and P. Diddy. "Billionaire" mentions Oprah, Angelina Jolie, Brad Pitt and President Obama. "I Like It" also refers to Barack and Michelle Obama, as well as Tiger Woods and Jesse James.

Bruno Mars is the only artist with three of the year's top 20 hits. His own smash "Just The Way You Are" is #13. Travie McCoy's "Billionaire" and B.o.B's "Nothin' On You," on which Mars is featured, rank #15 and #16, respectively. Mars will wind up as the first artist with three of the year's top 20 songs since 2008, when Rihanna had four of the top 20.

Eminem and Taio Cruz are the only artists with two songs each in the year-to-date top 10. Both are represented with one solo hit and one collaboration.

It's remarkable that a generation of pop fans will think first of Katy Perry and Bruno Mars when they hear the titles "California Gurls" and "Just The Way You Are." Older fans have vivid memories of hit songs with the same titles by the Beach Boys and Billy Joel, respectively. (But the new songs were substantially bigger hits. Both had long runs at #1. The oldies both peaked at #3.)

Here are the 20 songs that have sold the most digital copies so far this year. The tally after the title is the total number of copies the song has sold through the week ending Dec. 12. Some of these hits sold additional copies prior to this chart year (which began with the week ending Jan. 10). If a song ranked among the top 200 songs of 2009, I show its 2009 ranking and sales tally. At the end of each entry, I show the song's peak position on the Hot 100 and the number of weeks it held that spot.

1. Train, "Hey, Soul Sister," 4,154,000. This likeable tune is the group's biggest hit to date, surpassing its 2001 hit "Drops Of Jupiter (Tell Me)." "Hey, Soul Sister" sold an additional 687K copies in 2009, which made it that year's #131 song. Peak: #3.

2. Katy Perry featuring Snoop Dogg, "California Gurls," 4,152,000. This irresistible popsicle, which topped the charts in both the U.S. and the U.K., is the year's top collaboration. This will be the second time in the past three years that Perry has had a hit in the year-end top five. "I Kissed A Girl" was #4 for 2008. Peak: #1 for six weeks.

[Photos: Snoop Dogg volunteers at a community center in Manchester, England]

3. Eminem featuring Rihanna, "Love The Way You Lie," 4,003,000. This smash is vying for Grammys for both Record and Song of the Year. It's Eminem's longest-running #1 hit since "Lose Yourself." It ties "Umbrella" as Rihanna's longest-running #1 to date. Peak: #1 for seven weeks.

4. B.o.B featuring Hayley Williams, "Airplanes," 3,855,000. This was the biggest of B.o.B's three hits and the biggest hit ever for the lead singer of Paramore. This hit #1 in the U.K. Peak: #2.

5. Taio Cruz, "Dynamite," 3,663,000. This is the year's biggest hit by a male solo artist. This hit #1 in the U.K. Peak: #2.

6. Usher featuring will.i.am, "OMG," 3,633,000. Peak: #1. This ebullient smash, which hit #1 in both the U.S. and the U.K., is the year's biggest collaboration by two male artists. Peak: #1 for four weeks.

7. Eminem, "Not Afraid," 3,158,000. This anthemic smash was Eminem's first solo #1 hit on the Hot 100 since "Lose Yourself." Peak: #1 for one week.

8. Taio Cruz featuring Ludacris, "Break Your Heart," 3,146,000. Ludacris was added to the U.S. remix of the song. A solo version by Cruz topped the U.K. chart for three weeks in September and October. Peak: #1 for one week.

9. Lady Antebellum, "Need You Now," 3,084,000. This heartfelt ballad, a Grammy finalist for both Record and Song of the Year, is the year's biggest country hit. It sold an additional 1,325,000 copies in 2009, which made it that year's #57 song. Peak: #2.

10. Ke$ha, "TiK ToK," 2,913,000. This is the year's biggest hit by a female solo artist. It sold an additional 2,443,000 copies in 2009, which made it that year's #17 song. It has sold 5,346,000 total copies, more than any other song on this list. (Only six songs in digital history have sold more.) Peak: #1 for nine weeks.

11. The Black Eyed Peas, "Imma Be," 2,814,000. The Peas had the top two songs on the year-end Digital Songs chart for 2009: "Boom Boom Pow" and "I Gotta Feeling." Peak: #1 for two weeks.

12. Enrique Iglesias featuring Pitbull, "I Like It," 2,790,000. This genre-bending smash was Iglesias' first top 10 hit since "Hero" in 2001. It was the Latin/pop star's fourth top 10 hit and the third for the reggaeton rapper. The song borrowed a riff from Lionel Richie's "All Night Long (All Night)." Peak: #4.

13. Bruno Mars, "Just The Way You Are," 2,747,000. Mars proved he could fly solo with this sweet song, which hit #1 in both the U.S. and the U.K. (In the U.K., it carries the subtitle "Amazing" to differentiate it from Billy Joel's 1978 smash.) Peak: #1 for four weeks.

14. Katy Perry, "Teenage Dream," 2,676,000. Perry followed "California Gurls" with this terrific single. This will be the second time in the past three years that Perry will have two songs in the year-end top 15. Darren Criss' distinctive version on Glee also made waves. Peak: #1 for two weeks.

15. Travie McCoy featuring Bruno Mars, "Billionaire," 2,632,000. This charmer is the biggest hit ever for the lead singer of Gym Class Heroes, surpassing 2007's "Cupid's Chokehold," which also reached #4 on the Hot 100. Peak: #4.

16. B.o.B featuring Bruno Mars, "Nothin' On You," 2,604,000. This smash, a Grammy finalist for Record of the Year, reached #1 in both the U.S. and the U.K. It combines the old-school romanticism of Smokey Robinson with a modern, hip-hop sensibility. Peak: #1 for two weeks.

17. Ke$ha, "Your Love Is My Drug," 2,570,000. This is the year's third biggest hit by a female solo artist. Impressively, Ke$ha sings two of these three songs. Peak: #4.

18. Justin Bieber featuring Ludacris, "Baby," 2,568,000. This catchy tune was Bieber's first top five hit. It gives Ludacris featured billing on two of the year's top 20 songs. Peak: #5.

19. Far*East Movement featuring Cataracs & Dev, "Like A G6," 2,480,000. This is the year's biggest techno smash. Peak: #1 for three weeks.

20. Jason Derulo, "In My Head," 2,390,000. This hit reached #1 in the U.K. Derulo's previous hit, "Whatcha Say," was #10 on the year-end Digital Songs chart for 2009. Peak: #5.