Hippos, halos and black holes – the best scientific images of April

Ring of fire. The light swirls to the edge of a supermassive black hole in the center of the M87 galaxy, at 17 million parsecs (55 million light-years) from Earth. This image, the very first of a black hole, was published by the Event Horizon Telescope collaboration on April 10th. Collecting it was a gigantic undertaking and involved combining a value of petabytes of data (a petabyte was equivalent to 10¹⁵ bytes), captured by eight radio observatories around the world.

Underwater tours. A team of researchers from the Schmidt Ocean Institute in Palo Alto, California, discovered these mineral towers as they explored the hydrothermal vents of the Gulf of California. The towers, which measured up to 23 meters high, had outcrops known as flanges, pictured here, that contained overheated liquids. Despite the high levels of metals and sulphides present, the sites are bursting with life.

Glorious mud. This air shot of a hippo party, entitled "Hungry Hippos", won the top prize in the Nature category of the SkyPixel Aerial Photo Contest & Video Contest of 2018. It was shot by Martin Sanchez.

Life on Mars. An isolated astronaut peeks through sandstorms towards a base of Mars – or at least, a model of it. This is the Mars simulation base of the C-Space project, a teaching facility located in the Gobi Desert, near Jinchang, China. The base is divided into nine capsules, including residential premises, medical facilities and an entertainment and fitness room, and is open to school student visits to show what could look like life on Mars.

Photo in a thousand hours. It took 1,060 hours of exposure time to capture this spectacular image of the Large Magellanic Cloud, a satellite galaxy orbiting the Milky Way. And while it may look like NASA's work, the image was produced by a group called Sky Austral, five amateur astronomers from France. Between July 2017 and January 2019, the team observed the galaxy from the El Sauce Observatory in Chile and then assembled the images to form this unique high resolution photo.

Pollen Storm. This is the time of year when allergies start to be felt – think of those in Durham, North Carolina, at the beginning of April. A storm that hit the area raised a huge cloud of pollen, perceived here as a yellow-green haze on the city. The pollen was thick enough to be mistaken for a real cloud by a local weather sensor.

Strange lights. These strange ethereal lights appeared in the sky over Norway in early April, captured by a surprised passerby. Butit was not extraterrestrials – instead, the lights came from colorful gas detectors released by two NASA rockets as part of the Auroral Zone Flare (AZURE) experiment. A research team recorded the movement of tracers to determine how electrically charged particles move in the atmosphere at a dawn.

Halos in good health. Satellite images show 'halos' of bare sand surrounding the coral reefs of the Red Sea. According to studies published on April 24, these halos are formed by coral fish eating plants and invertebrates from the seabed. They are more numerous in marine reserves that prohibit fishing, as elsewhere.^1,2. Research suggests that halos could be a useful indicator of the health of the reef ecosystem.

Difficult situation. The police are trying to free the hands of an environmental defender stuck to the window of a train at London's Canary Wharf tube station. Members of the Extinction Rebellion group spent 11 days last month demonstrating in the streets of the city, including in the Oxford Circus and Waterloo Bridge neighborhoods. The group uses a non-violent protest to try to persuade governments to take stronger action against climate change.