— Tides and ga­lac­tic col­li­sions

The same force re­spon­si­ble for mov­ing oceans will also rip our galaxy apart.

Cosmos - - Contents - KATIE MACK is a the­o­ret­i­cal as­tro­physi­cist who fo­cuses on find­ing new ways to learn about the early Uni­verse and fun­da­men­tal physics.

SOUTH­ERN HEMI­SPHERE stargaz­ers have it good. From any­where on Earth, on a very dark night, the band of the Milky Way can be seen to stretch across the sky in a side­ways view through the disk of our spi­ral galaxy. From the south­ern hemi­sphere we can also see the part of the band where it widens into a bright bulge of stars, veiled by lanes of dust, sur­round­ing the su­per­mas­sive black hole at the very core of the galaxy.

Also from the south, due to the ori­en­ta­tion of the Earth and the So­lar Sys­tem, we can see the Large and Small Mag­el­lanic Clouds, dwarf satel­lite gal­ax­ies caught in the Milky Way’s grav­ity. One thing we can’t see, though, is the An­dromeda Galaxy.

Which is too bad, since An­dromeda, with its tril­lion stars and cen­tral black hole as mas­sive as 100 mil­lion suns, is hurtling to­ward us at 110 km a sec­ond.

Ga­lac­tic col­li­sions are com­mon­place in the cos­mos. Our best the­o­ries for how gal­ax­ies grow in­clude a healthy dose of can­ni­bal­ism, at least for the larger ones. Here in the Milky Way, as­tronomers (known in this con­text as ‘ga­lac­tic ar­chae­ol­o­gists’) have found long streams of stars trac­ing arcs and loops around the sky, il­lu­mi­nat­ing the re­mains of smaller ob­jects un­rav­elled by ga­lac­tic grav­ity as they fell to­wards us long ago.

The physics of how gal­ax­ies rip each other apart is the same as that which would be re­spon­si­ble for your grisly demise if you fell into a black hole, and it’s why Mars’s moon Pho­bos will one day be re­duced to a ring of peb­bles en­cir­cling the red planet.

It comes down to the tidal force: the un­even grav­i­ta­tional pull that hap­pens when one end of an ob­ject is closer to the source of grav­ity than the other.

The name ‘tidal’ is no co­in­ci­dence, as it is also re­spon­si­ble for why the oceans on Earth re­spond to the Moon.

The main ef­fect of a tidal force is to stretch and squeeze an ob­ject, elon­gat­ing it along the di­rec­tion point­ing to­ward (and away from) the source of grav­ity and squeez­ing it in the per­pen­dic­u­lar di­rec­tion. High tide on one side of the Earth cor­re­sponds with high tide on the op­po­site side, with low tide in the re­gions in be­tween. Sim­i­larly, if you fell into a black hole feet first, you would get much taller from the tidal stretch­ing, but also thin­ner, in a process vividly termed ‘spaghet­ti­fi­ca­tion’.

The tidal forces Pho­bos ex­pe­ri­ences from Mars are strong enough that the lit­tle moon will be broken apart in a few tens of mil­lions of years.

In galaxy col­li­sions, tidal forces can cre­ate long stream­ers of stars stretch­ing out across the cos­mos. When small gal­ax­ies fall into larger ones (which may one day be the fate of our Mag­el­lanic clouds), the stel­lar de­bris cre­ates thin faint arcs, trac­ing their fi­nal or­bit.

When large gal­ax­ies come to­gether, these streams can be flung out in tails thou­sands of light-years long.

The col­li­sions can be dra­matic in other ways as well, as ga­lac­tic gas coming to­gether can cause a burst of new star for­ma­tion and feed cen­tral black holes. Over time the cores of the gal­ax­ies spi­ral to­gether and the stars wash back and forth, blur­ring out the orig­i­nal struc­tures to co­a­lesce into an el­lip­ti­cal blob.

We see galaxy merg­ers all over the sky, and es­pe­cially in clus­ters of gal­ax­ies, where im­mense masses gather to­gether into one struc­ture. We also know, how­ever, that merg­ers hap­pen less of­ten than they used to.

As the uni­verse ex­pands, the dis­tance be­tween gal­ax­ies not al­ready tied to­gether by grav­ity is get­ting larger, so they bump into each other less of­ten. Over time, that will mean fewer stars, and a darker, lone­lier cos­mos.

Mean­while, we have An­dromeda. When it hits, in about four bil­lion years, it will be the big­gest light show our galaxy has ever seen. While stars will be flung about in dra­matic fash­ion, our So­lar Sys­tem as a whole will prob­a­bly be OK. The dis­tances be­tween stars are so vast that, even in a ga­lac­tic col­li­sion, in­di­vid­ual stars al­most al­ways sail right past each other.

By the time it hap­pens, the Sun will have al­ready neared the end of its life: ex­pand­ing to its red gi­ant phase, boil­ing off the oceans and doom­ing the Earth to an­ni­hi­la­tion. Per­haps life will have an­other van­tage point to watch from by then. Four bil­lion years is a long time, and the show will def­i­nitely be some­thing worth wait­ing for.

When the An­dromeda galaxy hits, in about four bil­lion years, it will be the big­gest light show our galaxy has ever seen.

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