The Whirlpool Galaxy (M51) and companion galaxy NGC 5195
This sharpest-ever image of the galaxy M51 (NGC 5194), taken in January 2005 with the Advanced Camera for Surveys aboard the NASA/ESA Hubble Space Telescope, illustrates a spiral galaxy's grand design, from its curving spiral arms, where young stars reside, to its yellowish central core, a home of older stars. The galaxy is nicknamed the Whirlpool because of its swirling structure.
The Whirlpool's most striking feature is its two curving arms, a hallmark of so-called grand-design spiral galaxies. Many spiral galaxies possess numerous, loosely shaped arms that make their spiral structure less pronounced. These arms serve an important purpose in spiral galaxies. They are star-formation factories, compressing hydrogen gas and creating clusters of new stars. In the Whirlpool, the assembly line begins with the dark clouds of gas on the inner edge, then moves to bright pink star-forming regions, and ends with the brilliant blue star clusters along the outer edge.
Some astronomers believe that the Whirlpool's arms are so prominent because of the effects of a close encounter with NGC 5195, the small, yellowish galaxy at the outermost tip of one of the Whirlpool's arms. At first glance, the compact galaxy appears to be tugging on the arm. Hubble's clear view, however, shows that NGC 5195 is passing behind the Whirlpool. The small galaxy has been gliding past the Whirlpool for hundreds of millions of years.
As NGC 5195 drifts by, its gravitational influence pumps up waves within the Whirlpool's pancake-shaped disk. The waves are like ripples in a pond generated when a rock is thrown in the water. When the waves pass through orbiting gas clouds within the disk, they squeeze the gaseous material along each arm's inner edge. These dense clouds collapse, creating a wake of star birth, as seen in the bright pink star-forming regions. The largest stars eventually sweep away the dusty cocoons with a torrent of radiation, hurricane-like stellar winds, and shock waves from supernova blasts. Bright blue star clusters emerge from the mayhem, illuminating the Whirlpool's arms.
The Whirlpool galaxy is located 31 million light-years away in the constellation Canes Venatici (the Hunting Dogs), the Whirlpool's beautiful face-on view and closeness to Earth allow astronomers to study a classic spiral galaxy's structure and star-forming processes.
Picha hii ya juu zaidi ya Galaxy M51 (NGC-5194) iliyochukuliwa mnamo Januari 2005 na Advanced Camera for Surveys kwenye Telescope ya Angani ya NASA na ESA, inaonyesha muundo mkubwa wa galaxy ya spiral, kutoka kwa mikono yake ya spiral, ambapo nyota vijana wanaishi, hadi kiini chake cha katikati cha manjano, nyumbani kwa nyota za zamani. Galaksi hiyo inaitwa Whirlpool kwa sababu ya muundo wake unaozunguka-zunguka. Sehemu yenye kutokeza zaidi ya Mzunguko huo ni mikono yake miwili yenye umbo la mviringo, ambayo ni alama ya yale yanayoitwa galaksi za mviringo zenye muundo mkubwa. Makundi mengi ya nyota yenye umbo la mviringo yana mikono mingi yenye umbo la kawaida ambayo hufanya muundo wao usionekane sana. Mikono hiyo hutumikia kusudi muhimu katika magalaksi ya mviringo. Ni viwanda vya kutokeza nyota, vikikandamiza gesi ya hidrojeni na kutokeza makundi ya nyota mpya. Katika Whirlpool, mstari wa kusanyiko huanza na mawingu meusi ya gesi kwenye ukingo wa ndani, kisha huelekea kwenye maeneo ya rangi ya waridi yenye kuunda nyota, na kumalizika na makundi ya nyota ya bluu yenye kung'aa kwenye ukingo wa nje. Wataalamu wa nyota wanaamini kwamba mikono ya Whirlpool ni maarufu sana kwa sababu ya athari za kukutana karibu na NGC 5195, galaxy ndogo, ya manjano kwenye ncha ya nje ya moja ya mikono ya Whirlpool. Kwa mtazamo wa kwanza, galaksi hiyo ndogo inaonekana kuwa inanyosha mkono huo. Hata hivyo, picha ya Hubble inaonyesha kwamba NGC 5195 inapita nyuma ya Whirlpool. Galaksi hiyo ndogo imekuwa ikipita karibu na Mzunguko huo kwa mamia ya mamilioni ya miaka. NGC 5195 inapopita, nguvu zake za uvutano husababisha mawimbi ya hewa katika disk ya Whirlpool. Mawimbi ni kama mawimbi katika dimbwi yanayotokezwa wakati jiwe linapopigwa majini. Mawimbi hayo yanapopita katika mawingu ya gesi yanayozunguka ndani ya diski, yanafinya gesi iliyo kwenye pembe ya ndani ya kila mkono. Mawingu hayo yenye unene hukosekana, na hivyo kutokeza nyota nyingi, kama inavyoonekana katika maeneo yenye rangi ya waridi yenye kuvutia ya nyota zinazozaliwa. Nyota kubwa zaidi hatimaye huondoa vifuniko hivyo vyenye vumbi kwa mtiririko mkubwa wa mnururisho, upepo wa nyota unaofanana na kimbunga, na mawimbi ya mshtuko yanayosababishwa na milipuko ya supernova. Makundi ya nyota ya bluu-kibichi hutokea katika machafuko hayo, yakitoa nuru kwenye mikono ya Mzunguko wa Maji. Galaksi ya Whirlpool iko umbali wa miaka 31 ya mwanga kutoka kwa nyota ya Canis Veneris, na kwa sababu ya ukaribu wake na dunia, wataalamu wa nyota wanaweza kuchunguza muundo wa galaksi ya spiral na mchakato wa kuunda nyota.