Grow­ing gal­ax­ies

Penticton Herald - - OPINION - KEN TAP­PING Ken Tap­ping is an as­tronomer with the NRC's Do­min­ion Ra­dio Astro­phys­i­cal Ob­ser­va­tory in Pen­tic­ton.

Th­ese evenings, if it is dark and clear, you should be able to see a fuzzy patch high in the east­ern sky. If you man­age to see it, you will be look­ing at the most dis­tant thing you are ever likely to see with­out a tele­scope. That fuzzy patch is the core of the An­dromeda Galaxy, a great spi­ral of stars, gas and plan­ets, about 2.5 mil­lion light years away, where a light year is how far light trav­els in a year.

Light trav­els at just un­der 300,000 km/sec, so a light year will be a very large num­ber of kilo­me­tres. To deal with this we have de­vel­oped al­ter­na­tive ways of writ­ing huge num­bers. One of them is to write a num­ber, fol­lowed by E, and then the num­ber of ze­roes. In this method, 1, 10, 100, 1000 and 1,000,000 can be writ­ten as 1E0, 1E1, 1E2, 1E3 and 1E6 re­spec­tively. We can write a huge num­ber like 315 bil­lion bil­lion as just 315E18. This makes it easy to write re­ally huge num­bers. A light year is al­most 1E13 km. The far­thest gal­ax­ies we have seen lie 13 bil­lion light years away, 13E22 km. Our Milky Way Galaxy is about 1E5 light years in di­am­e­ter, and the An­dromeda Galaxy is about twice as large, some 2E5 light years across.

With gal­ax­ies be­ing sep­a­rated by such dis­tances it seems im­prob­a­ble that one galaxy will ever pass close enough to an­other for a col­li­sion or even to grav­i­ta­tion­ally tug at it. How­ever, galac­tic col­li­sions do hap­pen; they are an im­por­tant part of how gal­ax­ies grow. It’s just that th­ese things hap­pen over timescales im­mensely larger than the span of a hu­man life. We need to con­sider times­pans of mil­lions to bil­lions of years.

For ex­am­ple the An­dromeda Galaxy is head­ing our way at about 110 kilo­me­tres a sec­ond. We will col­lide head-on in about four bil­lion (4E9) years.

When we ex­plore the depths of space with our tele­scopes, we find many ex­am­ples of gal­ax­ies col­lid­ing or pass­ing close enough to one an­other to grav­i­ta­tion­ally pull each other apart. How­ever, this is not as dis­as­trous as it sounds; this is how gal­ax­ies grow. We know that stars and plan­ets grow by ac­cre­tion: lit­tle bits of stuff stick­ing to­gether to form big­ger lumps. It looks as though gal­ax­ies grow the same way.

Soon af­ter the Big Bang, just un­der 14 bil­lion years ago, tem­per­a­tures fell to where hy­dro­gen atoms could form. Some­thing dis­turbed the young uni­verse, caus­ing lo­cal con­cen­tra­tions of hy­dro­gen to build up. Th­ese col­lapsed to form the first gal­ax­ies and stars. Over the bil­lions of years since, th­ese gal­ax­ies have moved through space, some of them col­lid­ing with one an­other.

A col­li­sion between gal­ax­ies sounds like a topic for the ul­ti­mate dis­as­ter movie. Th­ese col­li­sions do look dra­matic through our tele­scopes. How­ever, gal­ax­ies are mostly ex­tremely rar­i­fied clouds of hy­dro­gen, with stars sparsely sprin­kled in them. When gal­ax­ies col­lide, they usu­ally pass through each other. A be­ing on a planet or­bit­ing a star in a galaxy that is hit­ting an­other is un­likely to no­tice any­thing other than over mil­lions of years they would see their ver­sion of the Milky Way change. The main con­se­quence of the col­li­sion is a big­ger galaxy and a wave of for­ma­tion of new stars due to the dis­tur­bance of the gas clouds.

Un­less the col­li­sion is very fast, the drag­ging ef­fect due to col­lid­ing gas clouds can be enough to slow the gal­ax­ies so that they be­come or­bitally tied to­gether. They then col­lide over and over again un­til even­tu­ally they merge into a sin­gle, larger galaxy. There is ev­i­dence that our own galaxy grew by col­lid­ing with and merg­ing with other gal­ax­ies. The Large and Small Mag­el­lanic Clouds, two small gal­ax­ies only vis­i­ble from the South­ern Hemi­sphere, are likely to be its next two snacks. Saturn lies low in the south­west, get­ting lost in the twi­light. Bril­liant Venus lies close to Mars in the dawn glow. Mars is much fainter be­cause it is far away, on the other side of the Sun. The Moon will be New on the 19th.

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