---
title: "H1821+643"
url: "https://spacecatalog.org/object/h-1821-643"
release: "DR1"
licence: "CC-BY-4.0"
citation: "SpaceCatalog. H1821+643 [catalogue entry]. Data release DR1 (2026-08-21). https://spacecatalog.org/object/h-1821-643"
---

# H1821+643

H1821+643 is a quasar in the constellation [Draco](https://spacecatalog.org/constellation/draco), 4.990 Gly from Earth.

The most luminous quasar in the nearby universe, and unusual in sitting at the centre of a massive [cluster](https://spacecatalog.org/clusters) of [galaxies](https://spacecatalog.org/galaxies). Its output has to compete with the cluster's own hot gas, which makes it the clearest place to watch a quasar heat its surroundings.

## At a glance

- **WHAT IT IS:** Quasar
- **HOW FAR:** 4.990 Gly — A luminosity distance, from its redshift — how faint it looks. The light itself left 3.38 billion years ago.
- **WHERE TO LOOK:** [Draco](https://spacecatalog.org/constellation/draco) — The patch of sky it sits in, as seen from Earth.

## Properties

| Quantity | Value |
| --- | --- |
| CLASS | Quasar |
| DISTANCE FROM EARTH | 4.990 Gly — by luminosity distance from a published redshift, flat ΛCDM with H0 = 70 km/s/Mpc |
| CONSTELLATION | Draco |
| RIGHT ASCENSION (J2000) | 18h 21m 57.2s |
| DECLINATION (J2000) | +64° 20′ 36.2″ |

## Catalogue data

### WHAT KIND OF THING IT IS

What the object is in itself, rather than how it happens to look from here.

| Field | Value |
| --- | --- |
| Accretion state | quasar — a thin disc near its Eddington limit |

### WHERE IT IS AND HOW IT MOVES

Position, motion across the sky and the size it covers on it. These are measurements of the object as seen from here, so every one of them depends on where here is.

| Field | Value |
| --- | --- |
| Redshift (z) | 0.297 |

## Also known as

- Common name: H1821+643

## Source

[SpaceCatalog curated data](https://spacecatalog.org/releases) — CC-BY-4.0

How this was made: https://spacecatalog.org/methods#source-curated

## Cite this page

SpaceCatalog. H1821+643 \[catalogue entry\]. Data release DR1 (2026-08-21). https://spacecatalog.org/object/h-1821-643

```bibtex
@misc{spacecatalog_h-1821-643,
  author       = {{SpaceCatalog}},
  title        = {{H1821+643}},
  howpublished = {SpaceCatalog, data release DR1 (2026-08-21)},
  year         = {2026},
  url          = {https://spacecatalog.org/object/h-1821-643},
  note         = {Catalogue entry, data release DR1 (2026-08-21). Values from SpaceCatalog curated data.}
}
```

SpaceCatalog curated data — CC-BY-4.0

The page itself, with its pictures, the sky map and the observing tools: https://spacecatalog.org/object/h-1821-643