---
title: "Galaxy Zoo"
chunk: 5/5
source: "https://en.wikipedia.org/wiki/Galaxy_Zoo"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T03:49:12.688307+00:00"
instance: "kb-cron"
---

The properties of Galactic Dust have been examined in several Galaxy Zoo papers. The interstellar medium of spiral galaxies is filled by gas and small solid particles called dust grains. Despite constituting only a minor fraction of the galactic mass (between 0.1% and 0.01% for the Milky Way), dust grains have a major role in shaping the appearance of a galaxy. Because of their dimension (typically smaller than a few tenths of a micron), they are very effective in absorbing and scattering the radiation emitted by stars in the ultraviolet, optical and near-infrared. Although the interstellar regions are more devoid of matter than any vacuum artificially created on earth, there is matter in space. These regions have very low densities and consist mainly of gas (99%) and dust. In total, approximately 15% of the visible matter in the Milky Way is composed of interstellar gas and dust.
The study of dust in galaxies is interesting for many reasons.  For example, the dimming effects of dust need to be corrected for to estimate the total mass of a galaxy from measurements of its light. Standard candles used to measure the expansion history of the Universe also need to be corrected for dust extinction.
A catalogue of 1,990 overlapping galaxies was published in 2013, which had been collected by volunteers on the Galaxy Zoo forum using SDSS images. The abstract states: 'Analysis of galaxies with overlapping images offers a direct way to probe the distribution of dust extinction and its effects on the background light.' This catalogue was also used in a study of ultraviolet attenuation laws.

== Galactic bars and bulges ==

Some spiral galaxies have central bar-shaped structures composed of stars. These galaxies are called 'barred spirals' and have been investigated by Galaxy Zoo in several studies. It is unclear why some spiral galaxies have bars and some do not. Galaxy Zoo research has shown that red spirals are about twice as likely to host bars as blue spirals. These colours are significant. Blue galaxies get their hue from the hot young stars they contain, implying that they are forming stars in large numbers. In red galaxies, this star formation has stopped, leaving behind the cooler, long-lived stars that give them their red colour. Karen Masters, a scientist involved in the studies, stated: "For some time data have hinted that spirals with more old stars are more likely to have bars, but with such a large number of bar classifications we're much more confident about our results. It's not yet clear whether the bars are some side effect of an external process that turns spiral galaxies red, or if they alone can cause this transformation."
Spiral galaxies usually have 'bulges' at their centers. These bulges are huge, tightly packed groups of stars. However, using Galaxy Zoo volunteer classifications, it has been found that some spiral galaxies do not have bulges. Many galactic bulges are thought to host a supermassive black hole at their centers: however pure disk galaxies with no bulges but with growing central black holes were found. That pure disk galaxies and their central black holes may be consistent with a relation derived from elliptical and bulge-dominated galaxies with very different formation histories implies the details of stellar galaxy evolution and dynamics may not be fundamental to the co-evolution of galaxies and black holes. It seems that these bulgeless galaxies have formed in environments isolated from other galaxies. It is hypothesised that the black hole mass may be more tightly tied to the overall gravitational potential of a galaxy and therefore its dark matter halo,  rather than to the dynamical bulge component.
In September 2014, a paper titled: "Galaxy Zoo: CANDELS Barred Disks and Bar Fractions" was accepted for publication by the MNRAS. This was the first set of results from the Hubble Space Telescope CANDELS survey that was part of Galaxy Zoo 4. The study reports "the discovery of strong barred structures in massive disk galaxies at z ≈1.5 in deep rest-frame optical images from CANDELS". From within a sample of 876 disk galaxies identified by visual classification in Galaxy Zoo 4, 123 barred galaxies are examined. It is found that the bar fraction across the redshift range 0.5 < z < 2 does not significantly evolve.

== Galaxy mergers and interactions ==

(See also under Retired projects above.)
Galaxy Zoo Mergers was a Galaxy Zoo project started in November 2009 and retired in June 2012. There have also been a number of studies on galaxy mergers, among which was a survey of ≈3000, which presented "the largest, most homogeneous catalogue of merging galaxies in the nearby universe". This catalogue was spread over two papers and was a result of volunteers selecting likely candidates from Galaxy Zoo 1 and posting them on the Galaxy Zoo forum. Other papers that have used Galaxy Zoo data resulted in observations that include those taken by the Chandra X-ray Observatory.

== Literature ==
Lintott, Chris: The Crowd and the Cosmos: Adventures in the Zooniverse. Oxford University Press 2020. ISBN 978-0-19-884222-4

== See also ==

Amateur astronomy – Hobby of watching the sky and stars
Blueberry galaxy -  Small and very active galaxies.
Gems of the Galaxy Zoos – Astronomy project
List of astronomy websites
List of citizen science projects
Participatory monitoring – Collection of measurements undertaken by citizens
Wisdom of the crowd – Collective perception of a group of people
Virtual volunteering – Online volunteering
Zooniverse projects:

The Daily Minor Planet -  The Catalina Sky Survey's NASA-funded citizen science project
Asteroid Zoo – Citizen science project
Backyard Worlds – NASA-funded citizen science project
Disk Detective – NASA-citizen science project
Old Weather – Citizen science project transcribing historical weather observations recorded at sea
Planet Hunters – Citizen science project to find exoplanets
SETILive
The Milky Way Project

== References ==