WHY SCIENTISTS NEED YOU

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Citizen ASAS-SN shows you "light curves" obtained using the All-Sky Automated Survey for SuperNovae (ASAS-SN). ASAS-SN is currently the only survey to monitor the entire visible sky every night. Over the duration of our project, we have collected close to ~1 Petabyte of image data.

ASAS-SN currently monitors the entire night sky in the optical wavelength range. To make our observations scientifically useful, we limit the photons that we capture by using a filter. A filter selectively transmits light with specific wavelengths. ASAS-SN currently uses Sloan g-band filters which are "teal" colored, with an effective central wavelength of 480 nm, and a FWHM of 141 nm. Previously, we used Johnson V-band filters which are "green" colored, with an effective central wavelength of 551 nm, and a FWHM of 88 nm.

A light curve is a record of the brightness observations of a given star made over the years. Light curves are used to understand how a star's brightness changes over time (or not).

The brightness of most stars remain constant, however a small fraction show brightness fluctuations. These are the "variable stars".

ASAS-SN light curves enable the discovery and characterization of variable stars, whose brightness fluctuates over time. After analyzing the archival V-band light curves of over 60 million stars through machine learning techniques, we have discovered over ~220,000 new variable stars in the Milky Way and cataloged over ~600,000 in total.

However, we have since overhauled our network of telescopes and we can now monitor about 100 million Milky Way stars in the newer g-band ASAS-SN data. Compared to our archival V-band data, this new data set is especially promising for the discovery and characterization of new variable stars.

With the g-band data, we have made several improvements over our initial V-band data set. We are now looking at fainter objects in the g-band, and have improved our rate of data collection from ~2-3 days in the V-band to ~20 hours in the g-band, allowing us to study and discover more variable stars in greater detail than we did before.

In Citizen ASAS-SN, we have identified candidate variable stars in our new g-band data and we want you to classify their light curves!

There are close to 150 different kinds of variable stars known thus far, but that doesn't necessarily mean that every variable star neatly falls into one of these categories. While machine learning works well for the discovery of classical variable stars, without a human touch, unusual variable stars are easily missed during this automated process.

Help us identify the most unusual variable stars in the Milky Way!

We have routinely identified unusual variable stars in our data while searching for supernovae, and our long baseline (~5-7 years) light curves are ideal for the discovery of unusual variable stars.

Shown below is the ASAS-SN light curve for an "oddball" variable star dubbed "Zachy's star" after the discoverer and ASAS-SN team member, Zachary Way. This star has been compared to the famous "Tabby's star" (which was first identified through the citizen science project Planet Hunters on the Zooniverse), and is currently the subject of intense followup:

When looking at light curves, Citizen ASAS-SN lead Tharindu Jayasinghe also identified the most extreme heartbeat star ever known. In fact, our machine learning classifier incorrectly identified this as an eclipsing binary. Without a human looking at this data, we would have missed this amazing discovery!

Variable stars are rare, and this is even more so for the unusual variable stars like Zachy's star. As you go through our data, you may see light curves that could help us identify the most unusual variable stars in the Milky Way galaxy! We will use your classifications to carry out follow up studies on these objects to discern their true nature. You might even discover something that has never been seen before!

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Variable Stars

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The study of variable stars has a rich history. It has been suggested that the ancient Egyptians first noted the variability of Algol, an eclipsing binary, over 3000 years ago. Recently, it has even been claimed that aboriginal Australians observed the variability of pulsating red giants long ago and incorporated this discovery into their culture and lore. It is possible that many other ancient cultures noted the variability of bright sources without any record of it surviving to the modern day. The first modern discovery of a periodic variable was made in 1638, when Johannes Holwarda recorded the periodicity of the Mira variable Omicron Ceti. The number of known variable stars gradually increased to ~12 by 1786, ~175 by 1890, ~4000 by 1912 and ~28450 by 1983. In the modern era, wide-field surveys like ASAS-SN have collectively discovered over a million variable stars in the span of ~20 years.

Stars vary due to various reasons, but they can largely be categorized into variability caused by intrinsic and extrinsic factors.

1. Intrinsic variables: these stars vary due to changes in their physical properties. Pulsating stars like RR Lyrae and Cepheids are intrinsic variables.
2. Extrinsic variables: these stars vary due to external properties. Eclipsing binaries are a type of extrinsic variable.

This variability tree from Gaia Collaboration, Eyer, L., Rimoldini, L., et al. 2019, Astronomy and Astrophysics, 623, A110 highlights the vast diversity of variable stars:

This variability tree manifests in the light curves of these variable stars, as can be seen below:

Depending on the variability type, the light curves of variable stars will have different shapes and features. These characteristics can in turn be used to identify the variability type, simply by analyzing the light curves.

We have compiled a list of some common periodic variability types in the ASAS-SN variable stars atlas, which shows their phased light curves (including sonified versions)!

To date, we have characterized the V-band light curves of more than 60 million Milky Way stars using a machine learning classifier. Through this process, we have discovered over ~220,000 new variable stars, a considerable improvement of the consensus of these objects in the brightness range probed by ASAS-SN.

Shown below is a sky map of the ASAS-SN variable star discoveries, colored by their variability type. ASAS-SN is truly an all-sky survey as can be seen here:

The All-Sky Automated Survey for SuperNovae (ASAS-SN)

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The sky is very big: even in the present day, only human eyes fully survey the sky for the transient, variable and violent events that are crucial probes of the nature and physics of our Universe. We are changing that with our "All-Sky Automated Survey for Supernovae" (ASAS-SN) project, which is now automatically surveying the entire visible sky every night down to about 18th magnitude, more than 50,000 times deeper than human eye. Such a project is guaranteed to result in many important discoveries, some of them potentially transformative to the field of astrophysics---think about ASAS-SN as the "SSST" - Small Synoptic Survey Telescope, complementing LSST and other time-domain projects by frequently observing the entire bright sky. Bright transients, Galactic and extragalactic, discovered early by our high-cadence survey, are especially valuable, as they are easy to study using relatively modest size telescopes.

ASAS-SN currently consists of 24 telescopes, distributed around the globe. ASAS-SN first unit, known as "Brutus", which also happens to be the name of the Ohio State mascot, comprises of four robotic 14-cm telescopes deployed at the Hawaii station of the Las Cumbres Observatory. ASAS-SN second unit, named "Cassius", also consists of four 14-cm telescopes deployed in Chile. In 2017, we deployed additional 8 telescopes at two other LCO sites: "Cecilia Payne-Gaposchkin", deployed in South Africa, and "Henrietta Leavitt", deployed in Texas. In 2017, the 5th ASAS-SN unit, "Bohdan Paczyński", was deployed in Chile. Finally, in 2018 the 6th ASAS-SN unit, "Tian Shan", was deployed in China. All these telescopes allow us to survey the entire visible sky every night, and are making our network much less sensitive to weather conditions.

The "Bohdan Paczyński", "Cassius", and "Henrietta Leavitt" telescopes are photographed below: