{"id":62712,"date":"2024-08-24T07:20:00","date_gmt":"2024-08-24T07:20:00","guid":{"rendered":"http:\/\/ea332cfa-5881-44ff-99d9-2fc2082a691f"},"modified":"2024-08-24T07:39:46","modified_gmt":"2024-08-24T07:39:46","slug":"how-do-we-know-what-our-milky-way-galaxy-looks-like-when-were-inside-it","status":"publish","type":"rss_feed","link":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/rss_feed\/how-do-we-know-what-our-milky-way-galaxy-looks-like-when-were-inside-it\/","title":{"rendered":"How do we know what our Milky Way galaxy looks like when we’re inside it?"},"content":{"rendered":"

Artist’s impression of the Milky Way. Credit: MARK GARLICK\/SCIENCE PHOTO LIBRARY <\/p>

By Ezzy Pearson\n <\/p>

Published: Saturday, 24 August 2024 at 07:20 AM<\/p>

\n\n

The Milky Way galaxy is much too big for us to be able to send a spacecraft far enough into space that it could turn around, photograph our Galaxy and show us what it really looks like.<\/p>

The Andromeda Galaxy<\/a> is the nearest major galaxy to the Milky Way, and it’s about 2.5 million lightyears<\/a> away.<\/p>

So reaching a distance where we could capture an image of our own galaxy similar to those we see of the Milky is not currently physically possible.<\/p>

Artist’s impression of the Milky Way. Credit: MARK GARLICK\/SCIENCE PHOTO LIBRARY<\/figcaption><\/figure>

If you were sitting in the living room of a house, with solid walls around you, would you be able to tell what the rest of the house looked like?<\/p>

This is the problem faced by astronomers trying to map out what our Galaxy looks like while sitting on a planet within it.<\/p>

Buried inside its disc, all they can see is a few local stars before gas and dust shroudsthe view, blocking the light from the Milky Way\u2019s distant reaches.<\/p>

Yet despite this they have managed to not only map out our local neighbourhood, they can see right across town thanks to the largest survey of our Galaxy to date by ESA\u2019s Gaia satellite<\/a>.<\/p>

\"When
When inside your house, it’s not impossible to definitely say what it looks like from afar. So how can we know what the Milky Way really looks like? Credit: Aaron McCoy \/ Getty Images<\/figcaption><\/figure>

What shape is the Milky Way?<\/strong><\/h2>

To start such cosmic cartography the first question that needs answering is the morphology \u2013 the form, shape and structure \u2013 of the Galaxy we live in.<\/p>

We know from looking at our celestial neighbours that galaxies<\/a> can be elliptical, spiral, irregular or dwarf, but what is the Milky Way like?<\/p>

You can answer that query yourself the next time you\u2019re under a dark sky looking at our view through the Galaxy \u2013 the Milky Way<\/a>.<\/p>

With the naked eye it just looks like a diffuse cloud of light, but through a telescope it resolves into a band made from billions of stars<\/a>.<\/p>

\"From
From Earth, our Milky Way appears as a dense band of stars stretching across the night sky. Credit: Dafydd Wyn Morgan, Deadvlei, Namibia<\/figcaption><\/figure>

The Milky Way is our view through a huge plane of stars, showing we are sitting within a huge stellar disc.<\/p>

“From looking at other galaxies we know that most discs show some spiral structure,” says galactic astronomer Prof Robert Benjamin from University of Wisconsin-Whitewater.<\/p>

“Because the Galaxy is a disc galaxy it is almost certainly a spiral galaxy.”<\/p>

These other spiral galaxies show arms of stars and dust wrapped around a central bulge of old stars, so it was thought likely that the Milky Way was the same.<\/p>

\"Artist's
Artist’s impression showing where the Sun and Earth are located in the Milky Way. Credit: NASA<\/figcaption><\/figure>

How do we know what shape the Milky Way is?<\/strong><\/h2>

Proving the shape of the Milky Way has not been an easy task.<\/p>

The best way to find our Galaxy\u2019s spiral structure was to map out the location of the arms.<\/p>

Dense with dust, the arms are prime sites for stars to form, and are alive with bright, young O and B classstars.<\/p>

In 1958 astronomer William Morgan used these youthful stars to trace out the Milky Way\u2019s structure close to Earth.<\/p>

Beyond around 10,000 lightyears, the haze of dust throughout our Galaxy became too thick, blocking out the light from stars behind it and bringing the mapping effort to a halt.<\/p>

It was only when Jan Oort led the first large-scale radio survey a year later that the a map of the whole Galaxy could be drafted.<\/p>

\"Galex
A view of the Andromeda Galaxy, 2.5 million lightyears away, captured by the Galex mission. Credit: NASA\/JPL-Caltech<\/figcaption><\/figure>

This project, Benjamin explains, mapped hydrogen gas between the stars using the 21cm spectral line it produces.<\/p>

“That line can be seen across the entire Milky Way because the radio penetrates the gas,” he adds.<\/p>

As hydrogen is a vital ingredient in creating new stars, where the regions where the gas was thickest dictated where a spiral arm should be.<\/p>

The surveys showed that the structure was definitely present in the Milky Way, but what was less clear was where the arms actually lay.<\/p>

Though determining the direction of a clump of gas is easy, working out how far away it is proved difficult.\u00a0<\/p>

For these radio observations, the distance was determined using the Galaxy\u2019s rotation.<\/p>

\"Stellar
Stellar movement across the Milky WayGaia spacecraft, 13 June 2022Credit ESA\/Gaia\/DPAC<\/figcaption><\/figure>

Determining the Milky Way’s rotational speed<\/strong><\/h2>

Different parts of the Milky Way disc move with different speeds depending on how far out from the centre theyare.<\/p>

When objects are moving fast enough the light they emit gets stretched out, changing its frequency, an effect known as Doppler shift.<\/p>

This size of this stretch can be used to find the object\u2019s speed, which provides an estimate the distance.<\/p>

However, this method is notoriously uncertain. There\u2019s no way to tell if an object is moving at a certain speed because it\u2019s far away, or because random fluctuations mean it happens to be moving a bit faster.<\/p>

The result is measurements have an error margin of more than 20%.<\/p>

Despite these flaws, the radio surveys began to reveal the general shape of the Milky Way.<\/p>

\"Radio
A radio image of the Milky Way, captured by the Murchison Widefield Array, 20 November 2019 Credit: Dr Natasha Hurley-Walker (ICRAR\/Curtin) and the GLEAM Team<\/figcaption><\/figure>

Finding the centre of the Milky Way<\/strong><\/h2>

As well as the arms, the surveys found that stars were densest in the direction of Sagittarius<\/a>, suggesting this was the direction of the Galaxy\u2019s central bulge.<\/p>

The exact centre was thought to lie in the middle of a complex radio source 25,000 to 28,000 lightyears from the Sun.<\/p>

This was confirmed in 1974 when it was found to host a supermassive black hole, one of which is known to lie in the cores of many galaxies.<\/p>

\"Sagittarius
Sagittarius A* black hole at the centre of our Milky Way. Credit: X-Ray: NASA\/CXC\/UMass\/D.Wang et al.; Radio: SARAO\/MeerKAT<\/figcaption><\/figure>

Another layer of this central region was peeled back by the infrared COBE satellite.<\/p>

Mapping the entire sky between 1989 and 1993, COBE revealed an asymmetry in the gas of the Milky Way\u2019s core.<\/p>

This was the first hint that the Milky Way was not only a spiral galaxy, but a barred spiral.<\/p>

An elongated bar of stars extends through the centre of the Milky Way, at an angle of 27\u00ba from the Sun, with the two major arms spinning off from either end.<\/p>

\"A
A view of the centre of our Milky Way Galaxy captured by the COBE spacecraft, which operated from 1989 to 1993. The picture shows the brightness of the full sky in infrared. The bright line across the centre is interstellar dust in the plane of our Milky Way Galaxy. Credit: NASA, Michael Hauser (STScI), COBE\/DIRBE Science Team<\/figcaption><\/figure>

The Galaxy gets in the way<\/strong><\/h2>

Beyond this busy central region things begin to get a bit less defined, as the problems caused by the Galaxy getting in the way of itself only intensify.\u00a0<\/p>

“The area of greatest uncertainty is anything that\u2019s the other side of the galactic centre, but inside the Sun\u2019s orbit,” says Benjamin.<\/p>

“Anything within the Sun\u2019s orbit is very hard to measure with Doppler shift because it gets confused with stuff that\u2019s closer and that happens to be going at the same velocity.<\/p>

The main structures are identified. But exactly how to place them is still debated.”<\/p>

A type of star found in areas of high star formation, known as a maser, emits clear radio waves that can penetrate through the entire Milky Way.<\/p>

In recent years, a team has been using the Very Long Baseline Array to find the distances to these masers by recording their parallaxes \u2013 measurements calculated directly from geometry, making them much more accurate than those found via the Doppler method.<\/p>

\"Known
Known as parallax, the apparent shift in a star\u2019s position over time allows us to calculate its distance. Credit: ESA<\/figcaption><\/figure>

Another satellite is currently in the process of taking the parallaxes of not just a handful of bright stellar objects but over one billion stars, around 1% of the Milky Way\u2019s stellar content.<\/p>

Gaia\u2019s map of the Milky Way is the most accurate map of the Galaxy we have.<\/p>

The emerging atlas charts not only the stellar layout within the disc, but also the globular clusters<\/a> and stars that orbit high above the Milky Way\u2019s disc.<\/p>

“There are stars that move outside the plane, but are dominated by masses within the plane,” says Benjamin.<\/p>

“If you know the velocity of a star, you can usually work backwards to find out where the mass is, so kinematics [the study of movement] outside the disc will tell you a lot about what\u2019s going on inside the disc.”<\/p>

It\u2019s these stars that have helped mark out the limits of our Galaxy.<\/p>

Spotted 50,000 lightyears away from the galactic centre they denote where the arms of the Milky Way, and our map, come to an end.<\/p>

\"Gaia's
Gaia’s all-sky view of the Milky Way based on the measurements of almost 1.7 billion stars. Credit: ESA – ESA\/Gaia\/DPAC, CC BY-SA 3.0 IGO<\/figcaption><\/figure>

Does the Milky Way have 2 arms or 4?<\/strong><\/h2>

The uncertainties and huge error margins caused by mapping the Milky Way from within can often lead to controversy over their interpretation.<\/p>

One of the biggest debates in recent years has been over how many arms our Galaxy has: two or four.<\/p>

“If you were an alien in a galaxy far, far away, you would see four arms, because the arms that you see at visible wavelengths are really just showing the very bright blue stars near areas of star formation,” says research astronomer Ronald Drimmel from the Italian National Institute for Astrophysics.<\/p>

“The real question is if you looked at the old stars, where most of the mass is. That would be a two-armed spiral.”<\/p>

\"The
The heart of the Milky Way, as seen by NASA’s Spitzer Space Telescope, 9 October 2019Credit: NASA, JPL-Caltech, Susan Stolovy (SSC\/Caltech) et al.<\/figcaption><\/figure>

The two most massive arms of the Galaxy \u2013 Perseus and Scutum-Centarus \u2013 have many of these older stars, and their strong gravitational pull causes gas and dust to pile up between them.<\/p>

Two lesser arms, called Sagittarius-Carina and the Outer arm, don\u2019t have as many elderly stars to make up the mass, but are rich enough in gas to be bright with star formation.<\/p>

To confuse matters further, there are also many bridges and spurs jutting off the major arms.<\/p>

One of these \u2013 the Orion spur, originating in the Sagittarius arm and crossing the Perseus arm \u2013 is particularly relevant to us here on Earth; it\u2019s the location of our Sun and Earth in the Milky Way.<\/p>

However, data from the NASA Spitzer Space Telescope was used to infer that our Milky Way Galaxy actually has two arms, not four, and that is now generally agreed upon.<\/p>

This article appeared in the November 2016 issue of BBC Sky at Night Magazine<\/em><\/strong><\/p> <\/body><\/html>\n

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